Middlegame Tips and Planning
General Middlegame Advice (Exeter Chess Club)
Basic Middlegame Strategy (pdf)
Introduction to Chess Strategy
Improve Your Middle Game
Instructional Chess Games (Exeter Chess Club)
Alekhine's gun
How To Build An Attack
Be Sharp
The Razor's Edge
Focus
Hermann - Charousek, Budapest 1896
Chekhover - Rudakovsky, Moscow 1945
Boleslavsky - Lisitsin, Moscow 1956 (The game at Chessgames.com)