if your doing it properly, put an ad in the jcentre, asking for c.v's and a covering letter, this should get you a good few, you will know who stands out to you, bring them in for an interview, have a friend or someone else with you for second opinion, promise to get back to them and review with other person, a second interview wouldnt hurt either dont worry about wasting time at this stage, better to waste it now then when paying the wrong one.
ask q's about, ambitions and all the obvious stuff,(ideally you want someone with some but not too much!!)
important- how they feel about working outside, adverse weather, are they quick at picking things up, enjoy a challenge.
happy working on their own? stress this you dont want them whinging the time you do leave them on their own.
etc etc etc