When people ask me how I remember songs, I usually claim "I haven't a clue!". Memorising them is easy enough, I find. Like lots of those above, I get a set of words together and practice singing in the car - usually over a few days or a week. I then sing the song a few times in public, when the opportunity arises - quiet nights at the club, small hours of the morning etc. I frequently then lay off the song for a while and let it ferment in my head. Remembering them long-term is another matter. If I feel the song suits me and I get some satisfaction out of singing it, then it just seems to set down roots. As I'm sure many of you find, you get to a stage where you start off on a song without the faintest idea of whether you will get to the end - and if I stop to think about it - I won't! Songs seem to go in cycles for me. Out of a repertoire of about 2-300, maybe 10-20% are in regular use, another 20-30% pop out occasionally and a few get dragged kicking and screaming from the darkest recesses on request or when another song suggests they would be appropriate. The point is - the membership of each group of songs is constantly changing!
IF you can use crib sheets or something similar and still do the song justice - great! I've heard the great Frank Harte use a little dictating machine to give him the starting note - in the middle of an illustrated lecture! He has no compunction about it and obviously felt he could give a better performance as a result. Anyone who knows Frank's approach to key selection will understand!
Regards