You can do this several ways.

But probably the best way is the method given in a National Semi application note: Put two diodes between the "-" terminal of the LM34 and GND. This offsets the voltage by about 1V. You will also note that - because the diodes have a temperature coefficient of their own, you should also measure the voltage at the junction of the diodes and the LM34 and take a differential reading. Of course, this will take two channels of A/D.

Look for National Semi Application Note AN-460, and check out Figure 10.