Fill a hairline crack with concrete sealer. Clear rubble from a large crack with a steel brush. Widen the crack with a cold chisel; it should be wider at the base than at the top and at least 1/2 inch deep. Brush out the chips, then scrub the crack clean with a solution of TSP (trisodium phosphate) and water, following the recipe for heavy cleaning on the TSP package.
Using a trowel, pack the still-damp crack with masonry patching compound or with vinyl concrete patching cement. Although more expensive, the latter is easier to work with and has greater bonding strength. Cover the patch with wet burlap and keep it damp for a day or two.