After troweling the patch smooth, let it sit tight for a couple of hours, then cover it with a sheet of plastic (use bricks or rocks at the edges to weigh down the edges). Keep the area covered for an entire week, making sure to give it a spritz of water each day to keep it moist.
Concrete is one of very few building materials that actually likes water—and the more of it, the better. While you obviously don't want to make repairs when it's soggy out, a gray, overcast day is just right.
Here's a stop-gap repair solution if you're too busy to tackle the job right now: simply pack the crack with sand and seal it with waterproof tape—it will hold for the short term.
Have any dry cement left over? In its powdered form, cement is easily ruined by moisture, so don't keep large inventories of it on hand—instead, try to buy just the amount you need. Store dry cement off the ground in a dry place, covering unopened bags with a plastic sheet or tarp. Opened bags should be placed inside larger plastic bags, and small amounts can be stored in an old coffee can with a plastic cover. If bagged cement contains unbreakable lumps, toss the whole bag. If you have several unopened bags, pile them on top of one another and place them close together in side-by-side piles.
Congratulations! Your concrete looks like new again.