I've been playing guitar on and off for about twenty five years. I never noticed that I have a problem maintaining a consistent beat until recently when I began playing acoustic songs with a singer.

I started practicing with a metronome to help. Are there any other techniques or exercises to help.
You have already started the best way. Keep practicing with the metronome until the song happens without thinking.
what barefoot boy said. the metronome is the only way to improve timing really. Most of guitar playing is muscle memory, so just start out slow. If you mess up, you're going too fast. My music professors in college told me for every time you make a mistake you have to play it the right way twice to correct it. so just go slow. it probably feels tedious but you'll have it down in a week if you just stay at it.