Theres lots of ways, first of all you could use character map.
Or HTML which has too ways. either &(Name); or &#(Number);
http://www.webreference.com/html/reference/character/symbols.html
The useful list to remember (at least partially) would probably be (case sensitive, especially for the greek letters):
pi (π ), theta (θ ), fnof (ƒ ), sum (∑ ), radic (√ ), infin (∞ ), ang (∠ ), int (∫ ), there4 (∴ ), ne (≠ ), equiv (≡ ), le (≤ ), ge (≥ ), perp (⊥ ), plusmn (± ), deg (° )