Making satisfied clients useful.
WHEN TO ASK
After you resolved something for them. That is when appreciation is highest.
After a successful migration or launch.
HOW TO ASK
Directly. "If you have been happy with the service, a short note I could use on my site would be genuinely helpful."
Most people agree and never think of it otherwise.
WHAT MAKES A GOOD TESTIMONIAL
Specific. What the problem was, what you did, what changed.
"Excellent service" is worth little. "They migrated our site over a weekend with no downtime and fixed the email problem we had lived with for a year" is worth a great deal.
ASK A SPECIFIC QUESTION
"What was the problem before, and how has it been since?"
That produces something usable rather than generic praise.
REFERRALS
Ask for those too. "If you know anyone who needs this, I would appreciate an introduction."
A referral incentive can help, modestly.
USING THEM
On your site, with the client's name and business where permitted.
Ask permission explicitly.
WHY THIS MATTERS MOST FOR HOSTING
It is a trust purchase. Evidence from real clients is the strongest argument you have.