Make sure the person or company you use are registered with the relevant trade body relating to there industry. You could check that they are on the website of the trade body. In our case this would be the NICEIC website.
It's not unreasonable for you to ask to see the trades persons public liability documents to make sure they have the relevant insurance to be working on your property. They won't have a problem showing them to you if they are reputable.
We never ask for money upfront. Some companies may ask for a 30% deposit which is not unreasonable. If you are asked for money upfront or before the work is complete make sure you refuse.