It's hard to keep track of all the different rules for wish making in Dragon Ball, so here you'll find a guide for the different sets of Dragon Balls detailing what they can and can't do.

Earth's Dragon Balls (before Dende's Power-Up)
1. All seven Dragon Balls must be gathered in one spot.
2. The words, "Come out Shenron!", must be uttered for the Dragon to appear.
3. Only one wish can be granted at a time; however, multiple people can be resurrected with just one wish.
4. The wish cannot exceed Kami's physical power.
5. The wish cannot exceed King Yama's power of jurisdiction. In other words, someone who died of natural causes, or has been dead for over one year cannot be revived.
6. No one can be resurrected twice.
7. After the wish has been granted, the Dragon Balls will turn to stone and scatter across the Earth.
8. One year after a wish is granted, the Dragon Balls will become active again, and another wish may be made.
9. If Shenron is killed he can be resurrected. The statue within Kami's shrine that is a symbol of his life must be rebuilt.
10. If the creator of the Dragon Balls dies without an heir, the balls will become useless.

Namekian Dragon Balls
1. All seven Dragon Balls must be gathered in one place.
2. The Namekian words, "Takkaraputo Popporunga Pupirittobaro," must be said to summon Porunga.
3. Three wishes can be granted at one time.
4. A person can be wished back more than one time. However, a wish must be spent on each individual brought back.
5. After the wish has been granted, the Dragon Balls will turn to stone. Although never specified if the Dragon Balls scatter randomly after a wish is granted, regardless of whether they do or not, they are usually held by the elders of each village on the planet.
6. 130 days after all three wishes are made, the Dragon Balls will become active and another wish can be granted.
7. It is assumed that the wish cannot exceed Guru's physical power.
8. If Guru dies without an heir, then the Dragon Balls will become useless.
*9. After Muuri's Power-Up, multiple people can be wished back at one time.

Dende's Upgraded Earth Dragon Balls
1. All seven Dragon Balls must be gathered in one spot.
2. The words, "Come Out Shenron!", must be uttered for the Dragon to appear.
3. Three wishes can be granted at a time.
4. The wish cannot exceed Dende's physical power.
5. One wish can be granted at a time; however, multiple people can be resurrected with just one wish.
6. A person can be revived multiple times.
7. Although never specified if someone can be wished back once they've been dead for over a year, it is possible. However, no one can be wished back if they died of natural causes.
8. After the wish has been granted, the Dragon Balls will turn to stone and scatter across the Earth.
9. One year after a wish is granted, the Dragon Balls will become active again, and another wish may be made. However, if only one wish is made, the waiting period is 4 months. If two wishes are made, the waiting period is 8 months.
10. If Shenron is killed he can be resurrected. The statue within Dende's shrine that is a symbol of his life must be rebuilt.
11. If the creator of the Dragon Balls dies (or disappears) without an heir, the balls become useless.

Dark Star Dragon Balls
1. All seven Dragonballs must be gathered in one spot.
2. The words, "Come out Shenron!", must be uttered for the Dragon to appear.
3. One wish can be granted at a time.
4. The exact amount of power these Dragon Balls possess is unknown. However, they are extremely powerful. Other Dragon Balls cannot undo wishes they grant, and they may be able to grant wishes beyond the power of King Yama and Kami.
5. It is not known if someone can be resurrected twice, but it seems likely that it's possible.
6. After the wish has be granted, the Dragon Balls will scatter across the Universe.
7. One year after a wish is granted, the Dragon Balls must be gathered and kept together. Otherwise the planet where they were last used to grant a wish will explode.
8. If the creator of the Dragon Balls dies without an heir, the Dragon Balls will become useless.