Call the score after every rally, saying the server's score first. This helps you remember the score, and also reminds your opponents of the score -- reducing the chance of a dispute. In terms of understanding the scoring system, see this page from my rules article: the scoring system in badminton.
i often hear players saying that since they started on the right, and they are on the right again. therefore their score is even. however, i'll understand if the two doubles players had not switched sides. but once they have, how can it be even?
Yes, saying the score out loud is the most important thing. Some people seem to think it's "weak" to do this, and they'd rather show off how they can remember the score in their heads. But then you'll see four such people playing together, and they'll stop after a while and say "does anyone know what the score is?", a very amusing sight.
That used to work with the old scoring system (15 points): you could use parity (odd or even) to determine position (or vice-versa). With the current scoring system (21 points), this doesn't work. Consequently, it's now a bit more difficult to keep track of the score/positions than it used to be. This is the one bad feature of the new system. On the other hand, you don't need to remember "second server" anymore.
