Assuming you know the general rule of "player at the bottom area gets the advantage", mastering the top spar area is important as you'll never know when you'll lose the bottom area.

Avoid moving in straight lines too frequently
Diagonal movements are much more effective in sparring. In both dodging and making your movements unpredictable.

Don't stay too far away from an enemy when at the top
Unless you have a high spread gun, the enemy can corner you with ease by shooting horizontally. Get closer to him and make him shoot between vertical and horizontal. He needs to keep "guessing" where you might be by shooting horizontally and vertically.

Identify your opponents tactics and habits
There are so many strategies so figure them out yourself. Common tactics players use are: slowly cornering in, or frequent rushing. See if the enemy tends to shoot horizontally or vertically more frequently. If he shoots horizontally frequently, move above him and shoot down, or rush past him from above. If he shoots vertically frequently, then it's an easy opportunity to access the bottom area.

Make your patterns unpredictable
Sparrers with simple movement patterns sometimes lose the fight. Keep your enemy guessing where you might be. Be creative with your movement. The key strategy is being unpredictable.

All guns reload
Fighting a souizen? Wait till he shoots 12 times. Then attack him upon his reload or rush down to gain the bottom area. This strategy isn't very effective though with certain other weapons such as sten, ab8, or other fast reloading guns.