Here are a few:
- Make sure only the two types of trees you want to breed are in range of your bees. Pollination range is remarkably large, so you might have to clear out a few stray trees along the edges.
- Use bees with improved pollination speed traits. Aura bees are the best. For early game, Industrious bees are a good choice. Any type works though.
- Ensure that your bees have End, Flowers, Jungle, Mushroom, or Wheat as their flower type. Apparently none of the others (or at least some of the others, not sure I have the complete list) won't affect trees.
- Make a thaumium grafter and enchant it with Repair, and it'll last forever. Saves a lot of bronze.
- Specifically for getting to chestnut: Breed oak/birch for lime, oak/lime for cherry, lime/cherry for walnut, cherry/walnut for chestnut.
- Be patient. If you've gone a really long time without seeing any leaf block mutations, you may need to relog or travel to another dimension; there's a display bug in the version of Forestry that's included with the current Ultimate pack which makes mutated leaves sometimes look like normal ones until you do one of those things.
- If you're going for new species, you don't need to bother with the treealyzer. New breeds are always pure.