| I'll refrain from the "If you take away the concept of lands, it's no longer 'Magic'" debate...
Here's a solution similar to what's been said, but a little more elegant:
Make your deck as normal, but limit it to 40 cards, and no lands. Draw and play as normal. Instead of playing a "land", you remove any card in your hand from the game, and put a basic land that shares a color with that card into play. (From a seperate, out of the game source of basic lands).
This changes things a little:
1) First, the desired effect: No more mana screw/flood!
2) Multi-colored cards are still very good, as you can get a basic land of a choice of colors, but once you make that decision, it stays as that type of land. For example, if you remove a Lightning Helix, you can get either a Mountain or a Plains for it, but not both.
3) You can play a lot more expensive spells, since early on you can just toss them for land, and you KNOW you'll have access to 6 mana on turn 6 if you want.
4) Cards like Rampant Growth and other "search you library for a land and put it into play" cards should just grab them from the basic land pile. Cards that search your library for lands and put them into your hand would also work simply by going through the basic land pile and getting them.
5) There are bound to be some cards that just wont work with this variant.
Later!
Gruns | 
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