Mixing bass is really hard to specify, given there's so many different types of "bass sounds" in metal mixes. You could have that thin almost-above-the-guitars bass sound (for example spiral architect), a dirty and distorted picked sound of for example Bloodbath, a clean vintage-style tone of Martin Mendez of Opeth, etc. etc. etc. and spectral placing of the bass in the mix is crucial to getting those different bass tones. And as many have said, most of the time the secret behind such clean-yet-powerful guitar tones is the underestimated (many times slightly distorted on the high end to blend even better with the guitars) bass sound sitting modestly underneath. mic'ing a cab is an option if you have a really cool cab, but not neccesary, a Di'ed bass track is what most do (listen to Daylight Dies, split bass sound through two DIs (clean and driven) and such a nice little bass sound sitting beneath the huge guitars). About starting tips, many have already said:
-New strings is a huge plus (try additionaly mic'ing the neck somewhere after the 12th fret to get only the trebly sound of new strings then mix in a tiny bit of that in with the other tracks, can work wonders)
-pick or fingers is your choice but should depend on the tone, not because "real kvlt bassist use fingers" (stupid statement that bassist wannabees tend to say, a "real" bassist strives for the tone he wants)
-Compression is very common because the nature of a bass guitar tends to give the bass some "sweet spots" where some notes sound higher than the others which creates some unwanted dynamics, plus since bass is supposed to be below guitars and all other upper frequency instruments it should be tamed with compression (sometimes hard compression or even limiting, all depends on taste and situation) to keep it from unpleasantly poking above these instruments in some parts of the mix
-Reverb and Distortion/Drive is purely out of taste, I personally like some drive most of the time, but sometimes a clean but heavy bass sound can be just perfect for the song. Or in the opposite, a clean, trebly but heavily compressed pick sounding bass can work wonders given the right situation (Ashes of the Wake by Lamb of God, bass sound on that album totally owns. The album in general is one of my favorite metal productions ever). Now reverb depends totally on the feeling you want to give to the song, Im mentioning it but I don't see it is used very often on bass anyway, but you never know when it could full up a mix's low end (I used some for my band's demo, got the shitty bass recording sounding a bit more full and heavy)
Anyway, hopes these tips prove useful to you