i recently tried a friends mead.
it was very well made, smooth, no alcohol aftertaste. you could tell, unlike many of my early meads, that it was made with care. the problem was it tasted flat. i had other people over when it was opened and they made comments like, bland, not exciting, watery, etc.....

this is a simple pH problem.

mead is a type of white wine. white wines have very little ability to hide flaws. pH is a huge player in mouthfeel.
in beer, this isn't very important, but in wine, its a big deal.
pH gives wine the "zing", the acid is what makes your gums tingle slightly, gives you a big mouthfeel. but when youare actually making it, its a very minor part that is easily overlooked.

the best time to add acid to your wine or mead is pre ferment. it is much harder to change pH after fermentation due to the buffering ability of alcohol. so, take a reading, and add your acid before you add your yeast.

so, how do you know when to change pH? this is something you can eventually learn to taste, but it takes a whole lot of drinking. i am at a point now where i can taste a sip and come very very close to guessing the pH of a wine. i judge it by how much my molars tingle, but i'm weird like that.
starting out, you will need either a pH meter, (~50 bucks) or wine acid strips (5 bucks for a hundred). the acid strips are the standard paper things you dip in the liquid like we all used in high school chemistry class, except these are shittier and can only be read in full sunlight; if you try to take a reading on a cloudy day, you're screwed. still, the strips are much better than nothing and i recommend them if you don't want to purchase a meter.
actual meters work the best and i highly suggest purchasing one if you plan on making several batches of wine.

the pH for white wines is generally in the range of 3.2-3.6 with the more acidic being favored by prussians like myself.

as far as acid adjustments go, you can use citric acid, malic acid, acid blends, whatever. its personal preference and more advanced folks match the acid to the fermentable (malic for cider, etc.) but truly, most will never taste the difference.

but my wine came out flat, what do i do now?
not a huge deal, you can always add 1/2 tsp of acid blend to every bottle, or just mix your mead with something acidic when you go to drink it. sure, it's kind of hoosier to mix mead and sprite, but it sure does taste great! some may find a higher class option is to mix it with a splash of flavored tonic water. whatever your tastes, they are all delicious.

if you have any questions about acid in wine or mead, please address them here. i am happy to answer any you may have to the best of my abilities.