So I highlight desired text, right-click and choose copy. Paste into reply box - highlight and then click 'quote' icon.
That's probably the quickest and easiest option. You can also insert a whole quote into the Quick Reply box at the bottom of each page by clicking on the "Quote" button of each post, that would save you one page load if you're on dialup, though the Quick Reply box doesn't have a lot of viewing room.
If I'm multiple quoting from one large block of text I usually manually input the quote code myself, so at the beginning of the text, I would type
[quote]
which opens the quote, and then at the end of the quote I would type
[/quote]
which closes the quote. If you want to put the name of the person you're quoting then you would type
[quote author=Whomever it may be]
instead of just
[quote]
Although I think Emma's method is probably the easiest
My method is just another way