1 - Know what you can afford and how important it is to YOUR wedding
Pointless spending an hour or even a minute discussing something you neither want nor can afford, later on you may need something else from that supplier who now thinks you’re a time waster and ramps up the prices.
2 - Google is your friend, facebook is your enemy - research.
The chap wants £500 and a google search tells you the going rate is £450, perfect, this is your ammunition, a chap on facebook is offering the same deal for £100 and tells you he’s the best. Of course, it’s easy to be the best AND cheapest when you've no idea what youre doing and don't plan on being around for the wedding.
3 - Be complimentary
Telling the cake lady that her cakes are the best ever and your guests will love it and tell everyone about it, is a powerful bargaining tool. Telling her “it’s only a cake, why is it so expensive?” is unlikely to get you any discount.
4 - Be specific
“I need 75 of those, in cream and I need them on this date and collected the same night” is specific., the supplier can quote accurately based on it and build it into a work schedule. Lage tack ons or reductions are ALWAYS more expensive
5 - Be prepared to say no, and mean it.
It's not what you want, or it’s not what you can justify spending, say thank you and walk away. Of course if they follow you, you are in a better bargaining position. Just be up front, “I want this and can pay this” If the supplier can't do it, they can't do it!