I don't know if this is any help but you've got four options that I know of - I'll use setting a map with a 10deg west declination as an example:
1. Don't bother - in Blighty the declination seems to be in the 1-3deg west range now, and for setting your map that won't make much difference. Do resections, or go walking in the States however . . .
2. Use one of the rhymes as described above. So north on your map (the gridlines) goes to magnetic 350deg on you compass.
3. Use the manual declination scales in your compasses capsule - you line the NS lines in your capsule with the gridlines, but point the needle to the appropriate place on the manual declination scale as in this photo of my Silva Type 4 (Two things - the camera lens monkeyed around with the needle making it 5deg, and the capsule and bezel are misaligned by 2deg, one of the reasons I don't rate Silva compasses)
4. By far the best solution IMO, get a compass with declination adjustment. The two I rate most are the Suunto M-3G and MC-2G, the former a baseplate and the latter a mirror sighting compass. I prefer the baseplate compass personally. Recta do ones physically the same as them, but the capsule markings aren't IMO as good.
Heres how you set it (using the MC-2G):
Turn the compass over, and using a little key on the Lanyard, turn it so the gate/shed matches the declination on the scale.
Now the compass is set, using it is a doddle - the NS lines in the capsule line up with the map, and the red goes in the shed:
Someone mentioned global needles - I don't understand it too much, perhaps someone can explain it to me, but apparently there is a vertical component to declination that varies from north to south, meaning a needle will not balance correctly if used in the wrong zone, and will scrape against the capsule and stick. A global needle has a ring magnet in the middle (or so it appears), which stops that happening. What I do know is that it has the pleasant side effect of making the needle quicker to settle and more tolerant of being tilted, it's probably as big a plus on usability as the capsule being liquid damped, though why they don't market it as such I don't know.
Hope that helps.