If you have bumps in your lawn you need to stomp it, or lift the turf and remove some dirt. Possibly invite your fat auntie Esmeralda and ask her to have a picnic in the questionable area.
If you have dips, just add alittle bit of earth (less than 1/4" thick) every week or so over the season and it will soon level out.
Ofcourse, that is if its smaller than a football pitch