Another thing is, If you allready know at least one song in either the key of E or A, I mean know it GOOD like a classic rock song that you've listened to since being inside your mum's womb. You "inner ear" should not be far from the correct freq. For me, I always loved (still do) David Bowie's Suffrugette City. So in my mind I can always hear that first chord sliding into A by Mick Ronson. I'd then tune my A string to the song playing in my mind. Then I'd tune the other strings to that. It'll take some practice but I found that after a while, I was getting my guitar tuned pretty much on the mark.
good Luck!