speed training question

So now that I have been cycling hardcore for the last three months and feel I have a good base in I went out for my first speed workout today. I was wondering how the rest of you train and to what limit you do your speed. I tried a couple of things today. I went out and warmed up and did my first speed set at 27mph for 2 miles. After a rest I went to my next one and that was to push 30+ for as long as possible. Which was like 90 seconds. I went back and forth with this workout varying my speed for the longer sprint and for the faster sprint. Needless to say my last sprint I dug myself into the ground and held 30 for 3 minutes.... and then almost puked. How do you do speed work and do you set limits to yourself or just go all out. I found that setting a goal speed and time helped out a lot. Thanks a lot BTers