When I sing acapella my voice has a tendency to creep up during the song and I usually end a verse a semitone higher than I started. Is this an issue? Are there any good methods to prevent this?
It happens, even with choirs and banks of violins. Obviously practice helps, and a suggestion is that you sing along with recordings, and gradually make their volume lower and lower. Another is to sing along, and mute the recording every so often. You may find it's partly because the song is low for you, so your voice compensates by rising against your better judgement. Playing an instrument will generally be of assistance too - both with you singing, and also the fact that you make music in a different way.
See if you can hum the bass part, or just the bass notes of the chords, without modulating. I find that some people's difficulty with staying on pitch is losing track of the root.
When I was in high-school, listening to my walk-man ALL the time, I learned to modulate my voice by paying attention to how it feels instead of what I hear. Teachers didn't realize I had the tunes going.
And I had to change batteries often. :)
You might try singing while you listen to the music, but as you get really comfortable with how you're voicing when you have music, learn how your muscles feel; and when you sing acapella, sing to that feeling, instead of listening to yourself.
(Just a suggestion from a complete non-professional who's awesome alone in the shower :)
Most people creep down! Just do plenty of practice with an instrument doubling the melody. When the instrument drops out, hopefully you'll have got accustomed to maintaining pitch.