I've been playing on the 1.9 snapshots for several months now, and I've discovered some useful tips that have helped me use mending more effectively. Here they are:

1. The best way to get mending is to get a librarian villager that sells it for a cheap price. I have a villager that trades it for only 12 emeralds, and with a large sugar cane farm or another cheap source of emeralds, it's easy to get a double chest full of mending books. Based on experience, I have found that it's actually fairly common to find librarians that trade it. And it's often pretty cheap because it's technically a level 1 enchant. If you can't get villagers for whatever reason, the second best option is probably to use an enchanted fishing rod. I would discourage anyone from trying to get the treasure enchants by looking for temples/dungeons because this isn't nearly as effective and your world size will increase by a lot of you generate a ton of new chunks.

2. In order to repair a tool with mending as fast as possible, it needs to be the only thing with mending That you are holding/wearing. If you are wearing armor with mending, even if the armor is at full durability, it will decrease the rate at which the tool you're holding gets repaired. This is because when you get exp the game randomly selects a mending item you have equipped to repair. If the selected item is already fully repaired, the exp will go to your experience bar even if you are wearing/ holding another item with mending. thus, the quickest way to repair a mending item is to unequip all your other mending gear.

3. It is possible to go strip mining indefinitely using a mending pickaxe, but only if the pickaxe doesn't have silk touch and you're wearing non-mending armor

4. I recommend keeping a few copies of your favorite tools with mending at your base. that way, when one wears out you can replace it with a brand new one. When they're all worn out, you can go to your exp farm and repair them all at once.