Crabgrass is an annual grass that germinates from seed each spring when the weather warms up. It's very difficult to control after it sprouts, but relatively easy to do so before the seeds germinate. Apply a pre-emergent herbicide to the affected areas of the lawn in early spring before the soil temperature reaches 50 degrees F (often about the time that forsythias bloom).
Clover is a perennial broadleaf weed that is resistant to a lot of herbicides. But if you use a combination product that contains 2,4-D, MCPP, and dicamba once or twice a year (in spring or fall) you should be able to keep clover from overtaking the yard.
Another consideration: if the weeds consist of more than 50% of the lawn, you may be best off killing off everything with glyphosate (Roundup) and reseeding the area this fall.