If your a quite wealthy club then you should get your coaches and scout lvl up to 10 then scout for players that would fill in your weak points there quite good as you pay less then half there worth, they have low wages and usually a rating round about 80. I bought a midfielder who was 78 and after 2 and a bit seasons he's went up to 84 he'll probably be 87 by the end when I've done 3. So it's best this way as they improve quite quick.
Don't renew contracts after a season. When you are asked whose contract you want to renew tick the box for what they will get. Offer the same wage they already have and wait until the end of season to renew for the same wage instead of having to triple it at the start.