When a provider you've seen gets busted,  what should a hobbiest do to reduce risk to himself?   I assume her phone and laptop contents have been compromised.

Remember your Fifth Amendment right against self incrimination.

If LE calls you and asks you to come in to answer some questions, or starts to question you on the phone, politely explain that you intend to not answer any questions in accordance with those rights.  Do not fall for any line about how you are not a target, and that if you don't answer they will have to arrest you, etc.  It's all BS that they are legally entitled to use to get you to talk.

Do not lie and say something like:  "I have no idea how my phone number ended up on her cell phone."  You can get in real big trouble for that.

If this has happened to a provider you know, you have my hopes that LE doesn't go after her clients.

(still not a lawyer)

bought with cash & not linked to you.   You should use a hobby email too.  Now if they find her dead body, they'll get a warrent for the phone provider & ISP to trangulate position...  If you are just a client among many, warrants are doubtful on a provider bust.
If LE busts her...  they got a LEO into her room or she went into theirs...  and she said the magic words "sex for money"...  needed to arrest her.   They really don't need other clients to give them info.  
Now, if you use a big agency, who is the real target...  they'll be trying to follow the money & link the providers to the agency to bust THEM which is more serious charges.  
It is possible that screening info...  which should be deleted by the provider...  might not be.  She might have a "client list".  But it's not like the movie where she writes a "dear diary".   Occasionally such lists get released but more often, it seems they are suppressed since they typically have no proof...  and the politically connected on the list would not do to be exposed.  
I prefer to send screening info in a email or on the phone, not some form.  I believe that emails & phone messages get deleted more quickly.

if she was smart she didnt keep your contact info on her phone or keep your emails after your session together.

The few gents I remain in contact with are listed as very generic things on my phone - Dr's office, landscaping, pest control, dry cleaners, etc.

If LE were to ever bust me and take my phone, they aren't going to learn very much from looking at the contacts and I delete all text messages daily as well.

Where do you save your data so you know you've seen someone before and so that you can provide a reference?

If you keep any notes or contacts on your computer, I would suggest using encryption so that it's not easily accessible. There's lots of free software out there, like TrueCrypt, that will encrypt individual files or whole hard drives. With TrueCrypt, you can even have hidden drives, so no one even knows there's encrypted files. Depending on your operating system, encryption might even be built in. Some versions of Windows have BitLocker, and later versions of Mac OS X have FileVault. For additional safety, put the encrypted contents on a USB drive and keep the USB drive separate from the computer until needed.

Saw one of my favorites on Tues. Was at home surfing the web on my laptop with the news on in the background on Fri when I hear of a prostitution bust so I stop surfing to see what it was about. It was my girl in court!

I almost hurled!!!  The girl I saw a few days ago is now on TV in court for prostitution. I was calling all sorts of people with no idea what to do so all I could do was wait and see. Turns out they wanted her for hiring underage girls and drug charges. I think this was in 2007 or so. I'm guessing it is over since she has been in prison since.

Funny, since I started this hobby I notice people in the hotel parking lots and lobby wondering why they are there. I probably looked the undercover cop in the eye at some point that day because she was being followed that week.

merely having your phone number on hers isn't illegal? Yah so what I called this girl. Maybe my intent was to hire her but we never worked it out. No proof we ever met and if we did again they have to prove what we did and prove it was illegal.

Doesn't seem to be worth the effort for them to chase us guys.