The technical answer is that military score is computed as a function of the amount of goods (as in Small Arms, Ammunition, Liquor, Canned Goods, Clothes, etc.) that is consumed by all your armed forces for their maintenance.
More brigades = more goods consumed = higher military score
More ships = more goods consumed = higher military score
What should also be noted is that because goods are the metric, higher quality troops will yield higher military score. A nation with 10 Guards brigades will have a higher military score than a nation with 10 Militia brigades.
Obviously, a player should keep his army's composition a sane one: A hundred brigades of Cannons will consume more goods a hundred brigades of Infantry, but a hundred Cannons and nothing else will not be effective at actually fighting.
Finally, people recommend Dreadnaughts because A. there's no limit on the number of ships you can have, compared to being limited to a max number of Brigades by your Soldier POPs, and B. Dreadnaughts consume lots of goods and are expensive to maintain, and so will yield a lot of military score as long as you can afford it.
A final note: The Easy and Very Easy difficulty settings have a modifier that reduces supply consumption of your military, to make their maintenance cheaper. This has the nasty side-effect of actually making your military score lower than it should be, because the computation doesn't take the artificial reduction into account. Either play on higher difficulty settings or remove the supply consumption reduction from static_modifiers.txt
So, the long answer to improving your military score is to build the highest quality land army you can, up to your max brigade limit and with a sane composition, and then build the largest navy you can with the most advanced ships available to you.