What you're basically doing is intarsia I think. You need to twist the yarn so there won't be a gap. Check these sites for some help on twisting the yarn.
Scroll about halfway down to see this one
http://www.knittinghelp.com/knitting...ed_techniques/
http://www.thedietdiary.com/blog/lucia/508
http://www.sweaterscapes.com/intars.htm