Alternative Forms

ā before (voiced labials): b, p, f, v, m
usually before: c, consonantal i, d, t
ab before vowel or h
sometimes before (liquids): l, n, r, s
abs sometimes before c, q, t  (abs te;  cf. abs-tuli < aufero)
[Lewis and Short]
atque probably always before vowel or h
[Oxford Latin Dictionary]
ex always before vowels; more common in general
ē sometimes before consonants [especially liquids: r, m, v]
[Lewis and Short: see s.v. ex for certain regular expressions]
Note: in general the use follows the pattern of compound verbs, but Lewis and Short gives “ex lege” as regular, whereas compound verbs with stems beginning in “l” (e.g. eligo) regularly use the “e-” form, as would be expected before a liquid.
nec/neque: and . . . not;  [negative]. . .  nor
nec negative aspect more prominent
neque connective force more prominent
[Lewis and Short]
ut / uti: uti is sometimes used before a vowel (according to my research in Caesar BG).
genitive personal pronouns
partitive (part of whole) nostrum;  vestrum
multī nostrum many of us
objective (i.e. care for) nostrī, vestrī
cūra nostrī  care for us