<p>How can I set cron to run certain commands every one and a half hours?</p>

Is there a good reason why you can't use 1 hour or 2 hours? It would be simpler for sure.
I haven't tried this personally, but you can find some info here on getting cron to run every 90 minutes:
http://keithdevens.com/weblog/archive/2004/May/05/cron
An excert from the above link:
```
```
0 0,3,6,9,12,15,18,21 * * * <commands>
30 1,4,7,10,13,16,19,22 * * * <commands>
```
```