There's a mnemonic: Can Dead Men Vote Twice At Elections?
The first letters in this sentence indicate the meaning: Compass +/- Deviation = Magnetic +/- Variation = True (Adding Easterly).
This means to get from a compass bearing, I add Easterly deviations or subtract Westerly deviations get the magnetic bearing. From that, I again add Easterly variations or subtract Westerly variations to get the true bearing.
This is for the process known as correcting, going from least "true" to most "true." For the other direction, un-correcting, well, there is a slightly less safe-for-work mnemonic for that too: True Virgins Make Dull Companions, Add Whiskey.
Edit (clarification from comments, adding references):
Ideally, compass deviations will be at or near 0°, and for a hand-held compass in the field, can often be ignored. But these mnemonics were invented by mariners, and are useful for navigators of aircraft and sailing vessels, where there is other equipment that generates its own magnetic field and influences the mounted compass in the craft. Any good navigator takes the time to "swing the compass" now and then, and maintains a detailed deviation table for the vessel.
Also, declination and variation are both terms for the same concept. At first glance, "variation" seems to be the preferred term in navigation, and "declination" the preferred term in cartography and geography, but this is just my own observation, and I have also seen some references use them almost interchangeably. Here is another reference: (link).
Finally, here's an article (focused on sailing) that talks about the listed mnemonics and gives examples: (link)