1. Siren tears: In classical mythology Sirens were fabulous monsters, part woman, part bird, who were supposed to lure sailors to destruction by their enchanting singing. Naturally, the tears of these seductresses were deceitful.
2. limbecks: alembics. An alembic is an early apparatus used for distilling which usually consisted of two flasks connected by tubes. Alembics were associated with alchemy, and in Shakespeare's time many charlatans claimed to be alchemists who could create love potions and such.
3. applying: Used in the medical sense, as in "the nurse applied an ointment to the sore spot."
4. Still: always. saw myself: expected.
6. thought itself so blessed never: i.e., thought itself never so blessed.
7. fitted: driven by fits.
8. madding fever: maddening passion or illness.
9. O benefit of ill!: Here the poet begins a new section of the poem in which he reflects on what he has gained from his bout of frantic love-illness.
10. better is by evil still made better: i.e., the experience of evil always gives us a greater appreciation of the good. This is the "benefit of ill" referred to in the previous line.
12. ruin'd love: I think that the "ruin'd love" is the love that existed between the poet and and the fair youth before the poet drank the "Siren tears" of a woman.
13. content: i.e., what makes me happy.