What happened the last time you asked to have your rates increased or get a rush surcharge to apply or a short deadline to be moved?
In some cases the request was probably granted, but in the majority of them it was probably declined or ignored. Ignoring is the easiest answer, and the party ignored typically picks up the hint and stops bringing up the subject without need for direct refusal. Let's look at the refusals instead.
I doub they were actually flat out refusals, direct and rude, except perhaps for a couple of cases out of a hundred. Rather, they were polite and attempted to rely on arguments.
Arguments can be valid or not, but in any case they attempt to support the refusal with something. Data or emotions or a promise or threat but still something. They are not necessarily enough to covince you, they often show that you can't convince the other party easily. In some cases you decide it's not worth it, and you don't pursue your own arguments further. In other cases they leave it up to you to tell no to the potential collaboration that has been brewing so far.
You probably haven't often heard that your rates are ridiculous or exortionate. Rather, the agency (or client, in some cases) can't afford or can't offer you more than than a fraction of your rate. Possibly with a mention of the economic cirsis or how bad the market is in general.
In some cases the answer steers you toward declining the deal (so they don't have to) or accepting it on their own proposed conditions, e.g. by sending you or requesting from you a document connected with subsequent stateges of recruitement, or reiterating the request for your confirmation of the job they are offering.
Sometimes there's some talk about what their standard policy is or what the other translators accept.
Guess what. You too can talk about your policy or about rates and other terms your other clients accept. You too can use words such as: 'unfortunately', 'sorry but', 'I wish I could', 'while I value our collaboration', 'while I understand/sympathise with your situation/concerns/position' and so on, and talk about the tough market realities (which are not permitting you to hand out discounts liberally, for example).
You can also say that a proposed rate isn't leaving you much in the way of profit and that you need to pay some bills and put some cash in the banks, or that it is not a sufficient reflection of the type and amount of work you'd need to do.
You too can end with, 'please confirm this order at (…).'
Just no raw emotion but always arguments and perhaps without turning down the whole deal, only the scope of their proposal you can't accept, while providing a fair counteroffer (which places the ball in their court).
Whether they are apologetic or adamant in their own no's, you can learn the execution from them.
Edit (27 March 2014): You're also more respectful when you give good reasons for turning down proposals, offers and suggestions, rather than handing down a flat no without explanation. Kindness and politeness open may doors. It's hard to communicate properly without respect and understanding. Your clients will respect you more if they learn that you are fair and reasonable, not whimsical or petty, which is crucial to your (personal and business) dependability, not just to your being well-liked or not.