Out of heap space with simple updates

[Michael Lackhoff]

I wanted to try to do the daily update with XML updates (was mentioned 
recently as the recommended way) but got an "OutOfMemoryError: Java heap 
space" after 319000 records.
I am sending one document at a time through the http update interface, 
so every request should be short enough to not run out of memory.
Do I have to commit after every few thousand records to avoid the error? 
My understanding was that I have to do a commit only at the very end. Or 
are there other things I could try?
How can I increase the heap size? I use the included jetty and start 
solr with "java -jar start.jar".
After I ran into the error a commit wasn't possible either.
What is the best way to avoid this sort of problems?

Thanks
-Michael