Experiment with the prices.

If you sell out often, bump the price up one. Sell out again, bump it up again. Keep doing that until your crowds drop a little, then consider whether you prefer a slight extra cash or slight extra possession. Try and apply this for League, Cup and Champions League games. Don't do it with friendlies however as you'll have no one watching. At the end of the day, the more fans you have, depending on your stadium size, the bigger the chance of selling out.

Also helps if you can upgrade your car park as much as possible.

Hope this helps.