Vim digraph feature (ctrl-k) is very convenient for this. Digraphs are two-character mnemonics for symbols. They tend to be easy to remember and they are standardized in RFC 1345 [1].

Greek letters are always a latin letter followed by an asterisk. Arrows and comparison operators are what one would guess. Mathematical symbols generally make some sense:

    <C-k> a *     α
    <C-k> F *     Φ

    <C-k> = >     ⇒
    <C-k> - >     →
    <C-k> > =     ≥

    <C-k> 0 0     ∞
    <C-k> d P     ∂
    <C-k> I n     ∫

[1]: https://tools.ietf.org/html/rfc1345