SECRET STARS

You may have missed the area outside the castle, where Wario can break the black brick. There are two stars there, one requiring you to don the mario cap and use his balloon ability to fly to the other area on the map(located southeast corner). This was the last star I got, so maybe it's the same for you.

Or maybe depending on which star you're missing, most people overlook the door behind the mirror in the castle. Just a tip.

Did you kill all of the boos in the courtyard?

1. In the room where the painting for Jolly Roger Bay is, jump to a high
ledge on the right side (facing the painting). You'll enter an underwater
area; swim around and collect the 8 red coins for this star.

2. In the character switch room, jump into the southeast stain glass window
of Peach to reach the Princess's Secret Slide. Simply slide down to the
bottom and hit the yellow box for a Power Star.

3. Replay the Princess's Secret Slide and get a faster time than 21 seconds
to get a second star.

4. In the Goomboss Battle area, collect the 8 Red Coins to win a Star (see
that section in the walkthrough for the locations of these coins).

5. In the Goomboss Battle stage, after you go down the slide, head to the
right. You'll find a hill you can go up, then on the long platform, a star
switch. Step on it to make a star appear atop a cage. Run to where it's
located on the map, then wall jump (yes, you need Mario for this one) to
the top. Once there, walk over to the Power Star to get it.

6. In the Wing Cap stage, simply collect all 8 Red Coins (pretty easy to do,
look in the walkthrough).

7. In Bowser in the Dark World, when you get to the end by the pipe, you'll
spot a star switch. Hit it to make a star appear on a platform above the
tilting platforms before the hill. Go back down and tilt on one end of the
platform to make the other side rise, then quickly jump up from it to the
higher ledge (if you have Yoshi, just flutter jump from the base of the
hill to the first platform, then flutter jump from there). Either way, the
star is on the last ledge for you to get.

8. In the Bowser in the Dark World area, collect all 8 Red Coins for a star.

9. In the Rec Room, jump into the painting of the Sunshine Isles. There,
collect the 5 silver stars to get another secret Power Star.

10. In the courtyard where you enter Big Boo's Haunt, you'll find several
smaller Boos. Defeat each of them to get a red coin; collect all 8 for a
star.

11. In the castle's courtyard, break the blocks to the right and drop into
the hole to reach the Battle Fort. Collect the 5 silver stars there to
get a secret star.

12. In the Big Boo Battle, collect the 8 Red Coins to get a Power Star (the
locations are in the walkthrough for that area).

13. Go to the Big Boo Battle as Luigi. In the third room, hit the red box and
make him invisible. Now, quickly jump/flutter through the next room to
reach the final chamber. Run across the bridge and jump into the ghost
painting to run into a hidden star!

14. In the Hazy Maze Cave as Mario, get to the underground lake. Ride the
Nessie to the door on the ledge you can't reach by swimming. In there,
drop into the portal to reach the area Behind the Waterfall. Jump across
the ledges to the last one (it's the one with 2 red coins on it). Use the
flower from the red box to invoke Mario's floating ability. Then, simply
fly straight up to this hidden star.

15. In the Behind the Waterfall area again (see the previous listing here for
details), collect the 8 red coins (use Wario and hit the red box to get
his metal power) to get this star.

16. In the room where the portal to Hazy Maze Cave is located, there is a
Toad. Talk to him to receive a Power Star.

17. In the Bowser in the Fire Sea stage, collect the 8 Red Coins to get a
Power Star (check the walkthrough for details on that).

18. In Bowser in the Fire Sea, you must find a star switch. I highly
recommend you use Luigi for this one. It's on a high ledge above the
sinking/rising poles in the end area. Hit it, then jump to the other high
ledge up here. Backflip and copter across the lava area below, and break
the glass ball to obtain your Power Star!

19. In the mirror room on the second floor, enter as Luigi. Take a flower and
walk through the mirror. Enter the door at the opposite end of the room.
You will end up in an empty room with the Power Star sitting right there.

20. In the Chief Chilly Challenge, collect all 8 Red Coins to make this star
appear.

21. In the Chief Chilly Challenge stage, pick up the Wario cap when you find
it. At the path split, hit the red box to become metal, then take the
narrow path to the left past the wind gusts. Break the black block to get
this star.

22. Talk to the Toad standing directly across from the Tall, Tall Mountain
painting and he will give up this Power Star.

23. As Wario, go outside of the castle to the drained moat, and break the
black block by the waterfall. Drop into the hole to reach the Secret
Under the Moat. There, collect 8 red coins (4 are on ledges along the
steep wall, 3 are in the air by the moving platforms, and the last is by
the cage at the end). Get them all and the Power Star will appear inside
the cage. Use the Luigi cap and the Power Flower from the box to get it.

24. In the Secret Under the Moat area again, get the Mario cap and hit the
red box at the bottom of the hill. Float across the gap to the southwest
ledge. Hit the star switch there, then hit the red box and take the Power
Flower inside. Start floating upward along the coin path, constantly
tapping B to go faster. At the first ledge you reach, run to the right
and hit the second red box for another flower. Repeat this to reach the
top ledge, where you'll break the glass ball for a star.

25. Talk to the Toad on the third floor, to the right of Tick Tock Clock, and
he will hand you another Power Star.

26. On the third floor, jump to the high ledge on the right side of the room.
Drop into the hole as Mario to reach the Over the Rainbows hidden area.
Take the first red coin and hit the red box for a Feather. Triple jump to
launch, then fly to the different platforms (look at your map to keep
track) and collect the other 7 Red Coins. Note that for some of them, you
will have to land and use the cannons mounted on a couple of the clouds
to reach them (i.e. The one floating within the poles below one large
cloud, the small cloud below that, and the top of that large cloud). When
you eventually uncover all 8 coins, the star will appear on the large
middle cloud, so fly over to get it.

27. In the Over the Rainbows secret area again, fly to the larger pink
platform with the red Bob-omb on it. Talk to the bomb and he will prepare
the cannons for you. Now, take the Wario hat and jump into the cannon.
Look around and you'll find a cloud near you with a tree on it. Line up
the sights with that tree, and aim a bit above it, then shoot. With luck
you'll grab onto the tree safely, then all you have to do is break the
black box to retrieve this star.

28. After you catch all of the glowing rabbits, you will receive the key for
the final door in the character switch room. Open it to receive this
elusive Power Star.

29. In Bowser in the Sky, drop the lower platform below the first set of
rotating ledges (it's after the first piranha plant you meet). Hit the
star switch to make a star appear. Now, run the length of this platform
and hop up the ledges to the platform before the tilting one. Run past
the piranha plant and hop onto the spinning platform, then break the
glass ball for the star.

30. In Bowser in the Sky, collect all 8 Red Coins (see the walkthrough for
directions) to get this Power Star.