Check your pronouncing.
Accent is inevitable, but if you pronounce "ka" as "kay", or "chi" as "shi", well, it's just plain wrong.
Make sure the microphone and recording program you use record clear samples.
It's not a nice thing when you record all those samples, and they ended up uber-muffled or with random noises, right? *Hypocrite*
Some microphones might have noise suppression filters, but air can still get caught in some notes. Try to not sing too near or too directly.
Avoid unwanted vibratos.
All UTAU samples might be straight and clean. You don't need to record vibratos when the program already programs them for you.
Try to keep a constant on your notes.
Not only keeping your notes straight (that is, without shifts in a phoneme), but keeping all phonemes with similar notes help.
Always sing when you can.
Unless you can't hold a note, or your UTAUloid is product of voice acting, try to sing when recording the samples, specially if you want your UTAUloid to be similar to you.
Try to keep the samples with a constant volume.
I'm such a hypocrite, but yeah. Also, try to check your mike's volume; if it's too high, it won't give nice results.
Same reason as above; it worked with Kenta, it might have worked with Tree, but it doesn't mean it'll work with you.
Check initial results in UTAU before going on.
Or else, you'll be like me and hate your VB's result with all your guts... orz Record some samples (like vowels, n, k set, I dunno, not too much) and test how they'll sound in UTAU. If you don't like it, well... Remake it until you like it. *Nods, hyper hypocrite*
Avoid doing VCV without having practice with CV.
Believe in me, it's better. VCV is far too complicated and long to get right in the first recording.
And yes, I called microphone "mike". It's amusing this way XD