One: Look in the walkthroughs and hints section first next time.
Second: In the saiyan saga, when there are multiples battles at Kami's Lookout between the Z Fighters, knock your opponent off of the lookout and then fly behind the one big building in the center of the lookout. Make sure that if you fire a special attack at your opponent, it would have to go through the big building. You can also do it in the Majin Buu Saga when you are SS3 Gotenks. It is easier there because you can throw him off with up = x and then fly directly backwards and fire off you finisher. However, it is also harder because you have to beat Buu with a certain move. The dragonball will be one the pedastal in the center of the building you destroyed, but if you don't get it, it will roll off so it gets much harder to see.