Go early. Not just the day that the sale opens, but also the hour they open... That's prob'ly my biggest pet peeve is that the sales tend to open Thurs. or Fri. morning when I'm at work, so there are times when someone will advertise having something but by the time I can get to the sale after work, the item is already gone.
If you live in a fairly large urban area (it doesn't have to be that large, but just the largest city in 100 miles), there are some web sites that will list the sales in your area and may have a small description of the items they sell... These are nice to check on a smart phone while going around to different sales; I like to check the site when finishing up one sale to see where the next sale is at. They may even have a link to a map, which the smart phone may be able to use for GPS navigation (esp. useful in those residential areas that loop all over the place and all have similar names like tree names, etc.).
Bring plenty of cash and in smaller denominations. Some sales don't have a bunch of change, so they usually appreciate when someone can pay without needing change. Sometimes if they're really hurting for change, they'll give you a discount if you pay in bills that they're hurting for; I once was given $2 off because they were needing $1-bills and I had only 10 left (my bill was $12, so it was either I give them a $10 and 2x$1s or pay all 10x$1s and get $2 off).
Just like amusement parks, sometimes the best days to go are the days that have the worst weather. If it's raining a lot, fewer people will go out to the sales, so the sellers are more willing to make a bargain for the few customers that do show up.
You can usually get the best bargains on the last day of the sale. Although the good items are usually picked over by then, the seller is also more willing to let the remaining items go for less than what they were originally asking for. Some sales will actually post something like "1/2-off all items on Sunday".