I find that if I knit-transfer 3 or 4 stitches very tightly each round to the next needle it eliminates holes and loose, ladder-y looking areas.
you can use a dpn that is one size smaller on the area that has the cast-on edge starting stitch - according to something I read in one of Elizabeth Zimmerman's books (can't remember which one -sorry) it will not adversely affect overall gauge if you knit with 1 smaller DPN. I did that with my current project Cleaves
on the sleeves and it looks pretty darn good :D
Carol had a good tip too - that will work for particularly large areas as well. Don't worry - your hole-y areas/laddering diminish over time as you get more proficient with your dpns.