Yay, another person who wants to rise to the challenge. Here's a few tips that I have learned during the construction of my
WiP completely converted guard army. (links to pictures in my sig. Sorry, I like to advertise a lot)
1. Get all your
40k bits in one place. (i use some small drawers). Sort them if you want. Spend a long time sitting at a desk with all your bits, trying different combinations of bits and just trying to get inspiration. Some great conversions have been made by sitting down and just fiddling with the bits you have.
2. Vehicles are important. If you are going to get any, make sure you do a bit of personalisation to the bodywork or the details. It makes them look so much better. (I have a thread about my vehicles)
3. Look everywhere for inspiration. In
WDs, on the internet (dysartes.com has some great articles and typing something like 'imperial guard conversions' into google image yields some interesting results) and from your fellow gamers at stores and conventions.
4. Always have some simple little conversion you can do if all else fails. (which you have already got. Mine was to cut the thicker part of the barrel off every lasgun bar one or two which were converted in some other way so that they looked more compact and better for tighter combat)
5. Hobby saws are your friends. Pin vices are your friends. Scalpels are your friends (as well as Stanly knives, but they aren't as good). Try to get a scalpel which has a blade that looks more like a D rather than a really pointy one V as the ends snap off really easily. Always keep both types of glues around.
6. Lastly, don't be afraid. About trying out conversions and about taking a long time, I've been working on my guard army for nearly 2 years and I've only got 1406 points not counting the vulture. But I've got 42 converted guardsmen and 4 converted vehicles, all of which I think turned out really great. And finally, if you screw up while converting, there is always a way out of it.
EDIT: oh and
here's a link to a dysartes article which tells you how to model carapace armor. also, photobucket doesn't seem to be working. You can still view picture though.