Techniques for proving bounds on integrality gap in LP(SDP)
This is somewhat heavy machinery for what you want, but there's been a large body of work on techniques for designing ever more refined LPs (SDPs) that get closer and closer to the desired integer program. A good reference that reviews these approaches is by Monique Laurent: A comparison of the Sherali-Adams, Lovasz-Schrijver and Laserre Relaxations for 0-1 programming. 

Apart from that, I am not aware of a single good source of references: I assume you've at least perused the relevant chapters in Vijay Vazirani's book ?