The guitar solo is the peak of the performance. The solo can catapult the song into the top spot on the charts. So let me take a moment to explain some techniques for mixing a good solo.
Solos usually come in towards the end of a song, so there is some build up to the solo. Rhythm guitars are laying the foundation for the solo to be built upon. Don't let the rhythm guitars steal any thunder from the solo guitar. Automate an EQ low pass filter to roll off some of the rhythm guitar's shimmer. This will open up space in the audio spectrum for the solo guitar to shine a bit more brightly.
Pan the solo guitar off to one side. If the vocalist is doing anything vocally over the solo, pan them to the opposite side. Try to isolate the solo guitar as much as possible to make it pop out of the mix. The panning and EQ can all be automated and this will allow a smooth transition in and out of the solo.
Adding a delay to the solo is a great way to make it sound bigger. If you've got the solo guitar panned to the right, then pan the delay to the left. A quarter note delay is personally my favorite, but use a delay setting that compliments the music. Use filters to shape the tone of the delay and make it appear to drift off into the distance. Roll off the low end up to 400Hz and pull back the highs to around 9KHz. If your delay has a feedback setting, adjust it to taste for the echo effect you desire.
These techniques should give you a good starting point to make your solos shine and get all the attention they deserve. Please leave any comments or questions and I'll get back to ASAP.