There's a few things you can do.
Check the EQ settings for each instrument, and make sure you've got it optimized. If the guitar is lacking mids, for instance, a lot of times you end up increasing the volume to compensate. This often makes the low frequencies more prevalent, and those will cause clipping a lot sooner. To fix it try cutting the lows (say, 80 Hz and down gets a sharp cut) and bumping the mids a little (700-800 Hz range maybe). By fitting each instrument into its space in the sonic spectrum you often gain 'perceived' loudness from a recording.
If you really want to get a "loud" - like, commercial loud - recording, it starts on a track-by-track level. Optimizing compressors & limiters on each microphone to minimize transients etc. On the mastering level you get into multi-band compressors and more exotic stuff, which helps to get a tight low-end without washing out the highs or giving the track that "squishy" sound you get when overcompressing something.
On the stuff I record, things with the most pronounced transients - the kick drum, snare drum, and bass for my stuff - all pass through a limiter/compressor before getting tracked so that they don't peak off the mic inputs. The final mix goes through a mild limiter patch with a slightly scooped EQ applied to give it the 'loudness' effect, and finally through a BBE Sonic Maximizer to open up the sound a little bit more. At that point, depending on how the waveform looks, I'll play with the limiter settings and try and squeeze some more out of it without completely killing the dynamic range.
There's a lot to it...I've been using the same recording setup for about 10 years now, and the quality of what I'm able to do with it has increased 10-fold. It's a trial and error process with countless hours of burning CD's and listening to them in all kinds of environments. It's also important to make notes any time you change something so that you can either revert back if it sucks or remember it if it's good...there are a couple tracks that I've gotten a killer bass drum sound on, for example, and for the life of me I can't figure out what I did differently for those.