It depends on how dirty it is. Often I'm able to remove any dust with a Handheld Bulb Air Blower like this. Using canned air can create problems like pushing dust and dirt inside the lens. You want to remove any particles you can before using a cloth or lens pen. Be sure you have a clean microfiber cloth (like you get from your optometrist or you can buy online) and you can carefully wipe the lens if it still needs it. If stuff still remains, you can try using a liquid designed for lenses and then wipe, but be careful not to use too much. Finally, you can use a lens pen to remove fingerprints or other oils, but note that you need to replace them periodically and use them carefully so you don't end up scratching the lens. Hope that helps.
Also as mentioned above, if you aren't hand launching, using a small landing pad can help keep the lens clean rather than landing in the grass.