Because you may not have the map completely opened up,
I write the two digit North and East Grid (Large)numbers in the interior of the map. Saves you from having to look all over for the base UTM numbers.
If you do this then no mater how you fold the map you can find the MGRS or UTM reference locations with out unfolding.

I find using a declination compensating protractor the easiest way of addressing the G-M (Grid-Magnetic Angle) issue.
It is so quick, and error free. All boat shops and FBO stores for aviation have them, also on ebay you can find the military version. Most also have a longer base line than regular protractors.
With a regular protractor you can mark it with a preset line so it will read magnetic as well.
Some company actually sells something like this.

Some people draw red magnetic lines on maps (aviation charts have them printed in magenta)
The only problem with this is over time the secular variations (yearly changes) add up and you have to buy another map or carefully erase and redraw.

I don't prefer this, but like to make one line on the map which is magnetic so it is easily oriented with the compass.

In the Army we just used the LARS (Left add,right subtract) method, which assumes the item you want is either left or right from what you have (Grid and Magnetic usually, sometimes True)

The old Aviators (and mariners) memory aid:
TVMDC AW and CDMVT AT,
Here because aircraft and ships also have a deviation due to the residual magnetic signature of the craft.

True Virgins Make Dull Company Add Wiskey
True +/- Variation = Magnetic +/- Deviation = Compass Add Westerly

Can Dead Men Vote Twice At Elections
Compass +/- Deviation = Magnetic +/- Variation = True (or grid) Add Easterly

On land you don't have a deviation (that it consistent anyway).

Variation is the same as declination, but early navigators used that term for celestial navigation so it was renamed variation.