For sparring, the best drill is, well, sparring. Second best is shadow boxing, for the time of a complete round. Line drills utilizing the sparring combinations the student will spar with are also helpful.
For forms, practice is essential. Students tend to perform better when they understand the application of the movements; they are easier to remember, and they tend to perform the movements more correctly if they know the application. Forms drills include progressive (1st move, return, 1st and 2nd move, return, 1st-3rd move, return, etc. to end of pattern), slow (for technique), fast (to check memory of pattern), in a box (complete pattern inside a proscribed area, usually 2 stances square), hands only, feet only, stances only, blindfolded...