I need tips for baking at high altitudes?

I love to bake, and live in Mexico city. I have been told to increase water and decrease baking powder when baking, but its been 2 years and I�ve still had no success. I really want to bake scones, and all the recipes I�ve found online basically give the rough idea but say "trial and error" is the only way. I think I�ve tried and failed enough times now!!! HELP!!