When you solve the cross on the bottom layer, plan out what you are going to do. Do that before you start solving, and take as long as you need. When you find patterns or situations that come up a lot, figure out the best way to deal with them. Then, when you get that same situation next time, do it the same way.
As you get better, try to solve the cross in eight or less moves; you never need more than eight. Also, when you get better, don't look at the cross while you are solving it. Plan out all of the moves, then do them while looking for other pieces.