[chuggs]

For making sure that I have a layer completely transparent where it should be, I typically cheat and throw a dummy solid layer behind everything. Make it a high contrast color like red or blue and then the areas that I didn't completely erase stand out like a sore thumb. If I'm still finding specs in the final image that I don't want, I'll go back into the PSD file and give the transparent layer some absurd blending option like a thick solid outer glow or a dark drop shadow.

In regards to the transparent sig, keep in mind that not all forums allow gif or png files as sigs. For these forums, you can either view the page source or just take a screenshot of a typical thread and paste it in Photoshop to determine the background color that the forum is using. Set that as the background color of your sig. Below are a few that I've done with gray backgrounds to match the forums at FullSizeChevy.com. The only way anyone will notice the difference is if the forum allows different settings for each user that allows different background colors. Note that this can also cut down on your finished file size, which will make it more user-friendly to other forum members who might not have high-speed connections.