One of our local dinghy fleets scores the final result from each day (evening in our case) as a single score in the series. You get 5 firsts out of five races on one night that will count the same as someone getting 3 first out of three races on another night. That kind of answers your question but ignores the issue of excluded races completely! 
The number of race scores to exclude when calculating a final series result for a long series can be difficult. You want to reward those who come out a lot (and thereby encourage that). You can do that with no or fewer discards. On the other hand, folks have other things going on in their lives and it doesn't seem fair to punish them for going on vacation or occasionally getting stuck at work or at a family outing. That weighs in favor of more discards. It is a tough balancing act. Our local summer-long keelboat series excludes your worst 3 scores out of 20 races.
A DIFFERENT APPROACH
There is another approach to the problem. There is a scoring system that calculates a "winning percentage" (if I remember correctly, Greg Bemis called it the Percentage Point System in his classic book "Yacht Race Scoring"). Each boat's score at the end of the series is the total of the number of boats they beat in each race they sailed in divided by the number of boats the winning boat beat IN THE SAME RACES. So, your score is a percentage-type score. 1 or 100% means you won every race you sailed in. 0.5 or 50% means that on average you finished in the middle of your class every race that you sailed in. You can multiple by 100 to come up with a final score between 0 and 100 (which is probably better for PR purposes). In theory, you could have two boats with perfect 100 scores because they never sailed in the same races.
There are a couple of obvious problems with the Percentage Point System however (which is not to say you shouldn't use it since ALL scoring systems have "problems" of one type or another when used on a long series and the task of picking a scoring system for a long series amounts to figuring out which "problems" you can live with). First, interim scoring is difficult and competitors can have a really hard time doing "what if" analysis so they know where they stand. With computers now days you could recalculate and post after each race (which you probably would do anyway) and that might help to minimize that problem. The other "problem" is motivating participation. If a boat has a couple of good results after the first few races her incentive is to stay ashore as her results might get worse. That is usually handled by setting a minimum number of races to qualify for the series. But, if you set the minimum too low participation will suffer and if you set it too high few boats will qualify for the series and the others may become discouraged and stop coming out.
Overall, I think the Percentage Point System is pretty darn fair. With that said, we don't use it in our local summer-long keelboat series because we think the concept and calculations are just too complex for most competitors to understand. Instead, we calculate a percentage score for each race (actually between 1 and 10) and then sum the race scores at the end as a high point score. As mentioned above, we allow 3 discards over the 20-race series. BUT, since a DNF is worth 1 pt and a DNC is worth 0 points there isn't a horrific penalty for having to count a race you missed. With that said, it is still true that few boats trophy when they missed more races than the number of discards (and therefore had to count a DNC race).
Not sure that was helpful but it might at least give you some ideas and stimulate some creativity.
Art Engel
Calum Polwart wrote:
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Hi, this is off topic as we don't use sailwave for the overall results in this series but you guys are my best hope....
We have a "fun handicap series" - aim to get non-racers racing. Its very much about doing a race. it lets OODs get a chance to try setting a course and no-one minds if they do silly things (although the week I set a down wind start they did object! I put the letters up back to front on the board!)
Anyway - we have OODs running anything from 2 to 5 races depending on the weather, the numebr of boats on the water and the course they pick.
Its scored high points but not sure that matters.
Does anyone have any suggestions for a discard system from this type of event? i.e. if you ahve a really good day and win with 5 races on the same day and soeone else wins with 2 the next week is winning the five fair?
Calum