Don't be pressurised into having unnecessary work done and beware of 'special offers' which you'll get only if you sign up for them immediately.
Don't pick the first builder or plumber that you see advertised.
Do judge companies on past experience. Have you, your friends or your neighbours used a company that proved to be reliable on previous jobs?
Do find out whether the contractor is a member of a trade association that may give extra security should a dispute arise. Contact the relevant trade association to find a member in your area.
Do ask to see examples of work that the contractor has carried out or references.
Do ensure the contractor is established, with premises you can visit. Be wary if only a telephone number is supplied.
Invite three or more contractors to quote for the work that needs doing and don't forget to check whether or not the quote includes VAT.
Unlike estimates, quotations are binding. Check if they're only valid for a specific period of time. If not they'll be valid for a reasonable time. So don't take too long to begin deciding whether or not to accept.
Ensure all tradesmen know exactly what work is to be done, what preparatory work is to be included, when the work should begin and how long it should take, and how the site should be left; that is, clean and tidy with all rubbish removed.
The things agreed upon should be detailed in the quotation so that both you and the tradesman know exactly what's expected of each other.
Check that all items are covered on this, including any materials/make of product you specifically requested to be installed.
The contractor should have insurance which covers damage to your property and your neighbours. Check this. Also, ensure that either the contractor's or your own insurance covers death and personal injury.
Ask each tradesman who gives you a quote about guarantees, particularly any which are insurance-backed.

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