One-stop shopping is rarely advisable - while the biggest suppliers boast lengthy lists, no one is brilliant in all categories.
Using at least two suppliers not only means a potentially better list, it keeps both suppliers on their toes. Of course, using eight suppliers for a basic list is just not practical and quite unnecessary, so how many suppliers you end up with will depend on the type of list that you want (see creating a basic wine list).
Be clear about what you want Do you want your list to be basic and cheerful? Cutting edge and esoteric? Or showy and exhaustive? What price range are you going for? What sort of margins do you want to make? (see mark-ups and how to deal with them) How much stock can you hold? Answer these questions and you're ready to bring on the merchants.
Do your homework first Look at their lists, away from distracting sales pitch. Compare prices; look at whether the wines they are offering suit your kind of establishment; ask around - there's no substitute for word of mouth recommendations and hit the trade shows.
Taste it If what's in the bottle doesn't taste good to you (leaving aside your personal leanings towards particular grape varieties and regions) - whatever your level of wine knowledge - don't buy it.
Arrange a general tasting, and ask for samples for the big-volume section of your list, so that you can taste the wines against the competition. Taste them blind, so you're not swayed by any preconceptions. Granted, this assumes a certain level of wine knowledge, which hopefully you've got! And if you haven't, talk to your supplier.
Take up training offered by wine suppliers This service can be extremely useful. Ask them for notes, even ask them to test you and your staff. Make sure that the training that they offer is suitable for your establishment - do you just need the basics, or do you want to know about food and wine matching, and how to upsell etc? These sessions need to happen regularly to make an impact, and cope with staff turnover.
Check out delivery details carefully Do they offer same day delivery for six bottles, or is minimum drop 20 cases? The latter can lead to certain chaos if you're a small restaurant with not much space to store wine.
Make time to write your wine list and work with your supplier, who can do the layout for you if you're uncertain.
The more personalised your list, the more it stands out. Remember to make sure that all vintage, producer and regional details are included (with correct spellings) - South African Sauvignon Blanc doesn't tell the customer anything.