I wrote a thread about blurbs a while back but it's gone, so some tips:

Be understanding if someone can't blurb your book

Use an author's blurb if they put in the time/work

Some people (me!) don't do pre-publisher blurbs (meaning, you're using the blurb to get a deal)

If you cannot use a blurb on the book (it happens), let the author know and ask to use it in online promo

Uh, do not send a mass email asking for blurbs...? Unique, personalized emails are the way

Be clear about deadlines, then follow up

Be grateful/pay it forward

The big thing is BLURBS TAKE TIME. You're asking someone to read an early, not final version of your book and endorse it. That's a big ask and a big favor if completed!