Get a new oil filter, spark plug, (air and fuel filter if required)
Remove the old blade and replace it with a gator mulching blade
Or sharpen your old blade. I use a bench grinder on the fine wheel side.
Using a hand file is a fools task and takes too long, balance the blade on a nail that is hammered sideways into a wall. As long as it's fairly close you are good.
If you have a yard that is larger than 1/4 acre, consider picking up a used 48" walk behind mower from craigs or the paper. I obtained a used zero turn belt driven Ferris mower about 9 years ago at a very reasonable price and it decimates my 1/3 acre yard in about 14 minutes! Driving tractors are slow and inefficient for most lawns. Mine also came with a steel mesh bag that is wonderful for chopping up and collecting the leaves in the fall.
(not using the wrench leads to a sore hand)
I have found that by removing them physically before they can blossom and cast their seed - their proliferation has dwindled each year. Place the removed weeds into the trash quickly so that the buds don't scatter about. *Important: remove the dandelions before you mow, so you can spot the yellow flowers easily. Also, you won't cast about the seeds everywhere on your lawn.
When it comes to trimming - use an electric if you can or a small gas powered unit. Update 12-18-15: I got the Ryobi 48V from HD at beginning of the summer and mounted the pivot trim - it is phenomenal. (Though it did take a little shim to get the spacing to work out correctly) The fact that I can take it apart and throw in trunk is a real plus too. I highly recommend replacing the stock line feeder spinning head with a Pivottrim unit, it fits just about all trimmers. I also buy 100' spool of line, not the stock replacement pieces. Cut the line into 12" long segments with a sharp pair of scissors and thread them into the gator head. The rotating line holders all but eliminate line breaks due to the ingenious design. I have found this method to be the most efficient use of trimmer line and best performance. One set of strings lasts about 30 minutes or so, more if you avoid hitting the line against sharp edges.
One final note, I have used several types of gas cans in my life, the best one (for filling mowers only) is the style shown below by Blitz. Spilling gas is a big pet peeve of mine and these anti spill nozzles solve the dilemma. I even remove the nozzle and place it on a smaller blitz can with an oil/gas mix to fill up the trimmer tank - (they both have the same neck thread)