Nothing, I'm afraid. Even if you install a new ROM, the partitioning will remain the same. The standard partitioning for the Desire is 250mb for system, 40mb for cache, and 147mb for data and applications. Any ROM you flash has to fit inside that 250mb system partition, which is why HTC had trouble fitting the new version of Sense on the Desire.
You can, however, repartition the phone's internal storage by flashing a new HBOOT table. So, for example, if you're on the Oxygen ROM which is only 100mb, you can flash the Oxygen HBOOT table which reduces the system partition to 100mb and leaves you with a lot more storage for apps. However, I've always been too afraid to flash an HBOOT table in case I end up bricking the phone. It's not like flashing a ROM, where you can simply restore from a nandroid backup if anything goes wrong.