Lately, I've been throwing in anaerobic threshold sets into one day's swim. Anaerobic threshold sets are those where you swim to the absolute maximum level of effort at which the body can continue to transport enough oxygen to and lactic acid from the muscles. The workout sets fall into that space between aerobic sets, where you work out your cardiovascular system, and those where you're going all out using raw speed. During the anaerobic sets, your muscles do not get enough oxygen, and thus you're burning lactic acid, or something like that.
All I know is, these sets kill me. My favorite so far is:
1 x 50 easy
1 x 200 fast
1 x 100 easy
1 x 150 fast
1 x 150 easy
1 x 100 fast
1 x 200 easy
1 x 50 fast, repeat twice
I did the 200 fast on around 3:03. The 150 in 2:15-ish, and the 100 in 1:25. I rested no more than 15 seconds after each set. I get so freaking winded, I can barely breath. But I keep it up.
So now, besides my once weekly long swim (2+ hours), once weekly IM and once or twice weekly base/aerobic swim, I'm now doing an anaerobic day.