Another general principal

Always be expanding your ear:
By expanding your ear, I mean opening your ear, and this is generally the idea that I've been giving in the other posts anyway but I wanted to offer a few more ways to do that.
Opening your ear can be done a few different ways, usually simultaneously. The first way is listening differently than usual, that is, relaxing and listening for pitch colors or qualities. The next is changing up how you do that. By this I mean for example instead of listening to one note at a time, listen to two notes at a time. Instead of doing that randomly though, choose adjacent strings and do exercises similar to the ones I described for single strings. That is, play two notes, on the B and high E strings, and try to identify them. Listen to them a number of times for the pitch colors. If you are having trouble hearing the color/quality of one of the notes in particular, listen to just that note some more, maybe playing a few adjacent notes for comparison. Then move on to identifying two notes on the G and B strings, then the D and G strings, then the A and D strings, then the E and A strings, always listening more carefully to the notes where you have trouble and comparing them to adjacent notes. Then reverse the order. This is quite a good exercise and I am still doing it often to keep the ear opening up. You can try this skipping strings as well, e.g. playing two notes on the G and high E string, then the D and B string, then the A and G string, then the low E and D string. These exercises are good because they also give you practice listening to different kinds of intervals. As a result, your relative pitch will improve as well. The bonus is that when you go back (maybe the next day) to training one note at a time, it will seem easier to identify the notes. 
When listening to new things, the ear requires that it is relaxed so that it can open up more (Burge's mantra of don't strain!). Another example of incorporating something new could be using a different instrument. If you can get your hands on a bass, this will be especially helpful as a guitarist. It has a general timbre very similar to the guitar, so it's not too much of a stretch for the ear, but the lower range really helps your ear to open up. Again, try to relax with it. You can do exercises with the bass very similar to the ones I've described for the guitar. I would say to do only part of your training with the new instrument and then to go back to your main one, as then it will actually seem easier to identify tones on your primary instrument. I've found that I only have to do this once in a while to keep the ear opening up. 

Helpful Links

I haven't really gone over the basics of how to start training for perfect pitch, as I assume that most people here have given Burge's course a decent shot. If you don't want to get his course though, here are the links to the most helpful tips I've found online regarding starting out. They are primarily for bass players, but a lot of the principals will work for the guitar as well as other instruments. In general this guy just gives good ear training advice.

http://www.notreble.com/buzz/2009/10/22/the-zen-of-ear-training-part-1/

http://www.notreble.com/buzz/2009/10/29/the-zen-of-ear-training-part-2/

The only thing I would personally change about his advice is the part about using a metronome. It is very helpful to play tones in different rhythms repeating them a certain number of times, etc, (often I'll play a note and then softly count 2, 3, 4 in my head for example) but personally I would find a metronome distracting. I could see how some people would find it helpful though. Good luck as you continue training!