http://www.youtube.com/watch?v=U0G85zkc214
This is a simple technique I used when I put mitered borders on 4 quilt tops all in one day. Watch it a few times and see what pieces get lined up to make the 45 degrees.
Your border lengths should be about 4" longer than the adjacent borders combined with the quilt top. Remember to mark off 1/4" from the corners of the quilt top when sewing the borders on...you catch up to what you didn't sew when you sew the miter together.
Hope that helps - let us know if you need more explanations. Borders are a little tricky until YOU GET IT! Then they're easy!