Optimization/Level Design

Areas

Leaks: There's no excuse! Leaks will invalidate almost all of your efforts elsewhere.
Visibility: Reducing the number of surfaces and objects that are drawn in the first place. This is the largest and most important area.
Physics: Various tricks to avoid overloading the CPU with physics calculations.
Materials: Correct material choices will allow your map to scale down its demands on slower computers.
Lighting: Performance and file-size optimisation.

There are plenty more than those listed below — find them in their relevant articles. Note that sv_cheats must be enabled for most to work.

cl_showfps: A simple output of framerate. 1 is real-time, 2 is averaged over the past second.
showbudget: A panel which displays how your computer is spending its budget for each frame. It's the premier tool for working out exactly what's sucking up performance in your map.; It's invoked with +showbudget and -showbudget, which means that you can bind the former to a key (i.e. bind <key> +showbudget) and it will only appear when you hold that button down.
mat_wireframe: A console variable that lets you see through walls. This way, you can see exactly what is being drawn - for reasons described in the visibility optimisation article, often more than you might think is sensible.; There are three wireframe modes, 1 through to 3, which display the information you need with progressively fewer lines.