Generally "thinness" is something that happens more during the RECORDING process than during the mixing process. Get some better preamps, record more layers of tracks (double, triple and quadruple parts), get better microphones, learn to optimize your gain/SNR/signal levels and you'll find your mixes get fatter.
With that said, you DO need some monitors anyways. Otherwise you'll never get the stereo spectrum correct. Getting the volume levels mixed with headphones is no easy feat either.
But if your stuff is sounding 'thin' 90% of the time it has to do more with what you tracked and less with what you did with it. When I record I've already thought about the finished product--post mastering and mixing. So I record accordingly. If a snare has to be fat then I'm going to record the fattest sounding snare I can get: over mic, positioned overheads w/ a 3rd overhead for the snare, underneath the snare, plus triggers... and I'm going to use a high quality drum kit in a decent room. That is, if I have all those things at my disposal... I'm not tied to any particular, nor do I own my own studio, so what I get to work with varies each session.