Nice, crisp edging for beds

I put some fairly extensive beds into my yard. As the budget allows I'm filling with plant material. At some point I'd like to put in permanent metal edging, but can't swing it right now. In the meantime, what's the best way to maintain that edge? I see some lawns with a nice, clean, deep natural edges on the beds that seem to somehow prevent the grass from coming in. How is that done? Special tools or just a weed eater? Constant care? Any help appreciated.
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