There are selective herbicides which kill broad leaved plants like daisies while not affecting grasses. This would provide a 'quick fix' but daisies flourish on poor soil that is badly drained. You need to aerate the soil too by forking it and brushing sand into the holes, and feeding it to encourage the grass to grow more strongly.

You can also use a tool called a daisy grubber, you dig the the plant out with this. (bit of work to do if you have loads in your lawn like i have) supposed to get all the root out ?