There are dozens of ways to demarcate the border between your garden and your lawn, including decorative options such as bricks, stones or, for a seaside flavor, seashells. There are also premade edging materials available at garden centers and large retailers. These include molded plastic edging (that seems to have a propensity for popping out) and heavy-duty metal edging.
But the least expensive and most natural-looking edge is simply one cut in the grass. Establishing this edge is easily done with a gas-powered edger. If you're unsure about exactly where you want the edge to be, it's a good idea to draw line with landscape paint or sand (just put some sand in an old water bottle and pour it out in a line) so you have a guide to follow.
Set your edger to a good depth. The key to an effective grass edge is to make it deep and vertical, so that grass is less likely to continue to spread into your garden. Once you cut a nice, crisp edge, you'll probably have to go back and do a little cleanup on the garden side. Use a flat spade to cut off any pieces of sod that are still attached and compost them.
If you don't have a gas-powered edger, you can still have an excellent grass edge; it's just going to take a little more time. Using a flat-edged spade or a half-moon edger, cut a nice deep, straight edge. Pull out any sod you cut off and at the same time, mound your soil up into your garden, creating a V-shaped trough with a flat side. It can sometimes be helpful to follow up on your hands and knees with a trowel to make sure you have established a really good trough.
When you mulch your garden, do not fill in the trough. The hard work is establishing the edge, but after that's finished, occasional routine maintenance throughout the summer will keep its looks crisp. Trim back straggly grass with either a string trimmer turned so it cuts vertically or a grass shears. There are also long-handled shears available that allow you to stand while you trim the grass.
You'll be ahead of the game in spring if you have time to neaten up your edges one last time in fall, but it's a good idea to spend a little time in spring getting them looking really nice again.
After you've created a perfect edge you'll be shocked how good the garden and the lawn look, even if you've fallen a little behind on weeding.