My hair is naturally a dark blonde with lighter pieces

http://i1098.photobucket.com/albums/g365/dumblond01/hair/hair.jpg

But I dyed it burgundy red a few months ago. I also have to keep dying it every few weeks to keep the bottom half from fading.

http://i1098.photobucket.com/albums/g365/dumblond01/hair/hair2.jpg
http://i1098.photobucket.com/albums/g365/dumblond01/hair/hair3.jpg

I want to grow it out long again and I was wondering which would be least damaging.

Should I dye it my natural color now (and how would I go about doing this?) or should I keep up the red until it's grown out?

Your hair looked much nicer blonde!! I say grow it out though I suspect your hair will be stained and you will need to bleach it out to match which may make your hair a bit dryer. Go for the Ombré look!! Google it... its hot and edgy.