1990 with 35k miles?? Wow! I saw a convertible that looked in pristine original condition near my place about a week ago. I'm betting it's similar to yours.
Since you're doing mostly freeway driving, aero mods are going to be your friend. I'd also brush up on the driving with load (DWL) technique (see the driving tips:
100+ hypermiling / ecodriving tips to increase gas mileage - EcoModder.com ).
Since these cars are low on power, they don't do well at high speed Pulse and Glide (the pulse takes too long relative to the glide). But they kill at low & medium speed P&G.
Welcome to the forum!
Edit: OH - and look into installing an MPGuino for instrumentation.