Blender Tip #1, Axis Constraint

Most tools can be constrained to an axis by pressing either the X, Y, or Z key after you've activated a tool. This works for tools like G (grab/move), S (scale), and R (rotate), and there's probably more. One of my favorite uses for this is to be able to flip anything (ie: a group of polygons selected in the UV) exactly. To do so, select your object, press S (scale), X (to constrain to the X axis), then type -1 (to flip).