Honey Trees

As you travel around Sinnoh, you would have noticed strange yellow trees scattered around. These aren't just for show; they are used to attract wild Pokémon. (Think of the head butting tree technique in G/S/C, kind of like that, except slower. xP) The goal is to smear the tree with Honey and wait for a hungry Pokémon to come by to feed. Below is everything you need to know about Honey trees.

Basics

To begin, you'll need a yellow honey tree. There are many of them around Sinnoh so you should be able to find one without too much difficulty. If you're having trouble, here are some locations of Honey trees:

Route 205, Route 206, Route 207, Route 208, Route 209, Route 210, Route 211, Route 212, Route 213, Route 214, Route 215, Route 218, Route 221, Route 222, Floaroma Meadow, Fuego Ironworks, Valley Windworks and Eterna Forest.

Try some different trees until you find one that draws in the rare Pokémon the best. There aren’t any certain trees that have more rare Pokémon than others, it’s all luck.

Now that you have your tree, you need some Honey to smear on it. You can buy Honey from a Man in Floaroma Meadow for $100 or you can find them attached to wild Combee.

Once you've smeared your Honey on the tree, you’ll have to leave it alone for a while. Come back in about half a day to check up on it. If you come back too soon, the tree won't have changed. After the half day, your tree should be shaking. The magnitude of which it is shaking can give you some insight of the rarity of the Pokémon inside. The more violently it shakes, the rarer it might be.
Also Note: If you don't check your Honey tree within one day, the Pokémon will leave and be lost forever. Make sure to get back in time!

Alright, the Pokémon that comes out of the tree is pretty much random. It doesn't change with the amount of Honey you apply or anything like that. Obviously, some Pokémon are rarer than others. Here's a little guide:

Very Common: Wurmple, Silcoon (Diamond Only), Cascoon (Pearl only)
Common: Combee, Burmy, Cherubi
Rare: Aipom, Heracross
Extremely Rare: Munchlax