General Notes
A discussion of various post-hoc tests can be found at StatSoft and NIST.
A paper that summarizes and compares the various methods is attached.
(See "Stat Notes - ANOVA Post-Hoc Tests.pdf")
Depending on the objectives of the study, it may be worth considering pre-planned (a-priori) contrasts instead of post-hoc tests.
Assumptions
Listed in the paper attached below.
References
StatSoft Electronic Statistics Textbook
NIST Engineering Statistics Texbook
Milliken, George A., "Analysis of Messy Data Volume 1: Designed Experiments", Chapman and Hall/CRC, 1998, ISBN 0-412-99081-4, Chapter 3