Hiya I was thinkin you no Pokemon that pick up thinks while your walkin..

Hello. What a good question. This is how Pick Up works.

The items that a Pokemon with the Pick Up ability varies with the level of the pokemon. You can recieve items such a revive, super Repel, Super Potion, Nugget and even Rare Candies.

Having a lot of gym badges or the nature has no role in the process of getting items through a Pokemon with Pick Up. The items can be retrieved by battles and by walking around.

Also, the higher level your Pokemon gets, the less frequently you get items.

Your a man after my own heart! I LOVE the pickup abiliity-I got soooo much stuff!! In FR/LG mainly all you got to pick up was berries(I got a few nuggets, even a few TM-10's, etc. From my Meowth, mostly berries though)but I used Kutkat for HM's, too, dual purpose.

Zigzagoon/Linoone is perfect in Emerald for just that, plus he can learn more HM's than a Meowth, and won't lose pickup if he evolves!
The way you actually find stuff is:
1-occasionally just walking around with it in your party-not very often though...

2-after a battle there's often something left behind which your poke will then grab-but it's not related to what the foe Pokemon was holding or might've been, it's determined by the level of the pickup pokemon. According to Nintendo Guide, it's only really supposed to work if he's in 1st place in the party during the battle-but that's BS!!

I rarely ever have mine in 1st place and he gets me plenty of stuff all the time, even when he's not in the battle!! BUT, in Nintendo's defense, the chances of getting an item after battle does increase when it's 1st or in the battle, but NOT exclusively!

So, if you're greedy, put him 1st, otherwise just keep him in party & most importantly, check him after each battle(or like every 3 or so) to see if he got anything because he can't get anything else if he's holding an item! (Obviously-but check often is the point to get the most stuff)
As far as nature-I'm not aware of it having any effect on pickup, but a more aggressive nature may help(maybe naughty?-LOL).

Try it out with a few different ones, you may have something there! How many badges you have means nothing(as far as this is concerned, of course, congrats on the hard work-now just go wipe out Norman with a fighting poke quick and get your 5th and get Surf-I got my 4th & 5th back to back, just used a Heracross around Level 27-30, it was a breeze!)as far as determining what you'll pickup.

Apparently, the only thing that determines what you pickup is the level of the pokemon. Example: Your level 15 zig: after a battle it should almost always get a potion or antidote; often get super potion, great ball, repel, escape rope, or x attack; seldom get full heal, ultra ball, or hyper potion; and RARELY may get a nugget or king's rock.

As it gets leveled up the prizes get better-around the 70's & up even TM's(#1, 26, 44), leftovers, elixir are the rarer prizes, but the common ones are great, too,like hp up, pp up, protein, max revive, etc.

Now you have more info than you ever dreamed possible about the ins and outs of the pickup ability and how to use it best! Now go out there and don't leave home without your Zigzagoon and get lots and lots of free stuff!

Correct. Here is a diagram of items that can be obtained with the ability Pickup.