There's been a recent increase in developers and mod authors posting on threads and while there's plenty of room for everyone there's a few things to bear in mind.
1. Posting in a way that makes you sound like a third party (happy customer) to recommend your own customer is basically lying... Don't do it!
2. Not every thread is an opportunity to suggest your services. This is still a community forum where suggestions from other community members may be applicable. Use common sense and help where you can without feeling the need to say "I can fix this (for a fee)!"... A little pro-bono work will prove you know what you're doing and be a much better advert than just spamming.
This is primarily aimed at those offering custom work rather than specific modules but if you are offering modules please remember to read this...