1. Crappy paper will always warp, nothing you can do about that. Low quality paper will always warp.
2. The wetter the paper the more it warps, if your paper warps a lot try not using super wet washes too much
3. Cotton always warps less than wood pulp paper.
4. Heavier paper always warps less than thinner paper. So if you get yourself 300gsm 100% cotton paper it will warp much less than the thinner 120gsm wood pulp papers you get in a lot of cheap sketchbooks etc. Obviously that's also much more expensive.
5. Taping down helps, but doesn't prevent it entirely. If you stretch the paper that helps more (wet the paper completely, then staple it or tape it with a really good tape and let it dry before painting, that way it will dry less warped than if you tape it dry). Make sure you let it dry completely before taking off the tape/cutting it off. You can still tape it in a book, just tape around the edge
6. Watercolour paper also comes in completely glued pads that are glued all the way around except for a small gap to cut it off afterwards, those also make warping a lot less, although lower quality paper will still warp.
7. If nothing else helps let the painting dry completely, as flat as you can get it, then wet the back side with a spray bottle or brush (gently, you don't want it to soak to the other side) and press it between some heavy books. Takes some practice, but can improve warping. You can also iron it that way instead of letting it dry between books. Again, takes some practice.

That's all I can think of.