Make sure the wall isn’t damaged before or during anchor placement. If the wall is damaged, your anchor will fail and you’ll damage the wall further. They’re not suitable for use over patches or anything either.
Inspect the area before you put the anchor in. Poke around a little bit. Drywall can be damaged during the install and unscrupulous contractors will sometimes just cover it with mud and texture it over. This can be disastrous for you, and check for it first although it’s a very rare occurrence.
If you are trying to cover a patch, make sure that your anchors aren’t placed on the patch itself. To be safe try to get at least 2” of undamaged wall between the anchor and the edge of the patch.
You also can’t really trust the manufacturer’s rating for budget anchors either. There’s an incentive to make them seem stronger than they are, and you know what? Sometimes they won’t fail. That’s not good enough to risk a messy, time consuming repair to a wall however.
Always use the best anchors you can afford. If you’re going to go with plastic spread anchors or screw anchors try not to go over 75% of the rated load. This isn’t always possible, and it might be being overly cautious, but patching a wall is no fun.
Molly bolts are the way to go in almost every case. Unless you want to get into super expensive and hard to install toggle anchors, they’re the best as far as weight rating, durability, and ease of install goes.
Don’t “miss” when you put in an anchor, either. Actually, it’s better to miss by a couple of inches than by a fraction of one. You’ll need a sizable hole in order to put the anchor in, and you might have just messed up the desired positioning if you miss by only a couple of inches. Be precise.