On an earlier post, I wrote about baby pants and knickers. I have used Dana's Perfect Diaper Cover pattern and tutorial to make romper suits, too - works perfectly. You can find her pattern and tutorial here. Footnote 2015 - she has a revised pattern now but it is also on her website.
And just look at the fabulous material I found to make them! I called this the Gonk fabric when I was planning what to make.