I think the key is: play a lot. Cycle through your song book on a regular basis and refresh your memory. But that being said...
Learn the changes. Start with the chord progression and learn that first. Chart the song yourself. There's a massive amount of value in sitting down and writing it out by hand. First: you really have to listen and understand what you're doing to write it down. Second: you start to develop your own shorthand for what you write down, what you can remember and what you need to read from the paper and in developing that you learn things in a more permanent way.
Learn the melody. Being able to play what's being sung is the next part of understanding a tune. At least learn the key melody lines: the chorus outline, the verse outline. Again, you'll understand the song better and when it comes time to improvise you'll understand the theme of the song which, along with the chords, makes the song what it is.
Learn the hooks. Finally, learn the key bits of guitar that people remember. You probably don't need to know every bit of guitar, note for note (unless that's the type of band you're in). So learn the bits that matter.
The rest: fake it.
Though, truthfully, once you understand the chords and the melody you're not really "faking" it so much as "improvising intelligently". You'll find it's much easier to play what's right for the song when you understand it this deeply.