My 'primary' carry gun is a Ruger SR9c. I carry it in warm or cold weather. The only thing that determines whether I carry it or just my LCP is my environment. Both conceal very well for me. If I'm just going out to get gas or something, I'll sometimes carry my full size M&P OWB with a forward cant, but under a jacket of course.
If I am in a less than permissive environment such as church, a friend's house, or working on someone's property I'll usually only carry my LCP in my pocket. The reason is if my shirt happened to raise or get caught and show my firearm, there might be an issue.
I am 5' 7" 170lbs and I carry my SR9c IWB in a Kholster holster with a forward cant at about 4 o'clock. It conceals very well with very little to no printing in just a t-shirt. I use the flat base-plate on my 10rd magazine.
My LCP rides in a cheap nylon pocket holster and as you imagine, conceals very well in my pocket. I also prefer the flat base-plate on my carry magazine.
When carrying IWB, I try to limit the amount of bending over or reaching I do and I am conscious of how my clothing such as my T-shirt is riding. I try not to worry about it too much. 99.5%+people don't look for guns. As long as you don't look paranoid and you aren't messing with your belt line, you're probably alright. Another thing is too often overlooked. You need a decent carry belt. It should be thick and at least 1.5" wide. This will help keep the gun near your body much better than a $10 Wal-mart belt. I bought mine at a local fair from a leather-craftsman.