If I don't have a print out of an offical scorecard, I write on a piece of paper that goes into my wallet:
home team away team
T C P DG T C P DG
Then I box off each section with a big line on the intersection between teams. I then use the tally marks to keep a record of each score. At half time I try to add up the scores and start with a new set of boxes for the second half. At the end of the game I make the final score. Reverse side of the paper I keep a record of subs and a record of cards with the titles: Team, Colour, Number, Reason.
I also write above the name of the team winning the toss who kicks off in the first half, using the code KO1.
A foolproof system, until both teams come running at you at the end of a very closely fought game asking the score, and you have this:
Team 1 - 3 tries, 2 conversions, 1 penalty
Team 2 - 4 tries, 1 conversion
And you say : 24-22 to Team 1 because you've counted their 2 conversions as 6 points instead of 4.
Ooops...