Preventing Fleas in Your Yard
I'm assuming that you more or less have a flea free yard and that you want to keep it that way. If you have an infestation, then you need to see the page about treating your yard.
Fleas hate a well-kept, sun-drenched yard. They love areas where leaves pile up, where the grass is overgrown, and so on. What this means is that you will want to clean up the yard a bit. Mow it, rake it, get all of that junk out of there. Pay special attention to the area where the dog spends most of his time or where other animals might wander through. Don't worry if you have a big yard because for flea control purposes you really only need to worry about the area where your pet hangs out. So while it won't hurt to clean up the front yard, it won't affect your fleas if the dog spends his days in the back. Just getting rid of the hiding spaces can go a long way to keeping your yard and your pet flea free. Personally I'm too lazy to use a bag on the mower but I think if you are doing a flea clean up, it wouldn't hurt to suck up as much of everything as you can, including whatever fleas, eggs, and larvae you might mow over.
Another issue is that of stray and wild animals. Out here we have feral cats all over the place and they bring all kinds of fleas. You might have racoons or wild dogs or possums or whatever. In any case, you'll want to take whatever action is necessary to keep these varmints away from your yard. Most of the time that is just a matter of making sure there is nothing for them to eat but those cats wouldn't go away until we got a big dog.
Please don't use poisons as a preventative. If you feel the need to treat the yard with something, try using beneficial nematodes. Keeping your yard tidy etc. is the best preventative anyway.