If you need to do this a lot, you might get ukelele and use it to create your own keyboard layout. That’s what I did, and now I can type all kinds of mathematical symbols with ease, such as lots of integral signs (∫, ∬, ∭, ∮), ∅, ∇, ∀, ∃, ∑, ∏, ⊂, ⊆, ⊃, ⊇, not to mention arrows: →, ←, ↔, ↓, ↑ and so on. (However, you probably don’t want my keyboard layout, as it is based on the Norwegian keyboard.) I admit it took me a while to put it together, but I feel it was worth the effort. Your mileage may vary, however.