I'm new to offset smoking as well.
As as been mentioned above, there are several modifications you can/should make to your cooker to help with temp:
- Seal any leaks in firebox and smoke chamber with high temp. RTV and/or BBQ gasket
- Extend the chimney to grate level inside your smoke chamber
- Deflector and/or tuning plates to help heat dispersion inside the smoke chamber
Are you measuring temperature at grate-level? If not, you may want to relocate your thermometer to give you a better idea of the temp at grate level where your meat is cooking.
I like to regulate temps with fire size. I leave both chimney and firebox doors open to let my fire run as well as it can, and regulate my cooking temps with the size of my fire - this has worked well for me, as it keeps the fire burning clean.
It's going to take practice, practice, practice.