2.4 Use a Metronome
I was discussing about this site with my friend who goes to the 'Academy of Contemporary Music' the other day and the first thing he said was make sure you mention to use a metronome, and I have to say I agree.
A metronome comes in very handy when practicing as discussed previously (2.1 Practice, and start simple). It allows you to increase the speed at which you play a song in a systematic manner. This is critical if you ever want to be able to play fast as it will allow you to gradually increase your speed without increasing the risk of picking up bad habits.
When learning a particular solo or anything technical, you should start by playing it at a very slow tempo using the metronome. Once you are happy that you have got the hang of it at that tempo you should gradually increase the tempo with small increments, practicing at each set tempo until you are satisfied with it. For example you could choose on 5bpm increments.
It is also very useful if you ever want to keep a record of your progress (speed at which you can play a riff etc...)