One trick I used to save money, and which I shared with a couple of friends of mine who got married shortly after I did, was to purchase the stone and the ring/setting separately. If you go into a brick and mortar store you get a good deal on the setting and they gouge you on the stone. If you go online (BlueNile) you get the best deal on the stone and crushed on the setting. Buy them separately and have the brick and mortar do the mounting. Then let them tune (clean, reset, etc.) it up every six months. This keeps the warranty in force and heads off possible damage, loss of stone, that sort of thing. The store might not like you killing most of their markup, but they'll ultimately take your business to keep getting you into the store every six months in the hops that you'll buy something else.
I'm lucky in that my (current) wife is more interested in a future than more shiny baubles, so there's no pressure for me to buy more stuff, or her for that matter.
The other nice thing about using an online vendor for the stones is that you get a much wider selection. The online shops give you grades for the five Cs, detailed maps, and graphic images of the stones from many angles, so you can really go to town picking out something you really like (and at the best price point).
I think there is usually a step up in price around the obvious carat sizes (~.5 carat and 1.0 carat) so if saving money is of critical importance you might want to buy just under those threshholds.
For me it was very important to optimize on color and clarity. I've seen two-carat rings that look like soda water with all the bubbles in them. What's the point? The first time around I got a good deal by getting a stone that actually looked great but had a significant flaw but only along a single plane that could only be seen from a single angle and which could be well-hidden by the setting. As it turns out, my first wife had a similar, critical flaw, but that's another story. I'm sure she'd have some quality things to say about me, most of which are doubtless true. I'd like to think we both learned something from the experience. In the end there aren't any shortcuts.
Get the right girl. Do it right. Do it once. Save money, but get a nice ring that you like. Sacrifice size for quality. If you want to make it look like there's more going on you can go with a different cut (e.g., princess cut as opposed to one of the standard brilliant cuts) that might make a smaller stone look a little bigger. You can also go with side stones or a bunch of small channel stones in the setting. You can think of prices of stone increase with the square of the volume, so a stone with twice the volume would cost more than twice a much as a smaller stone. You can do a nice ring with a half-carat center stone and a quarter-carat companion stone on either side. Still one carat in total, but cheaper than a single stone of that weight.
Visit some jewelers. Ask to look at some stones in different colors and clarities. Get a feel for what you're looking at. Look at them through loupes and microscopes. Make conversation. Be nice. The salespeople will be helpful if they're any good. If they are good, even if you don't buy a stone from them, reward them by buying the setting from them. Any intended would love to hear all the details of what her man learned and how he did it all for her, even if he forgets all the details shortly thereafter.