I'd suggest three things;
1) Buy a new spa cover. Covers only last about 5 years. After that they tend to sag (or worse, rip in half). Since we're also in the market for a new cover, I've checked out some web sites. Most charge between $250 and $350 for a decent cover (up to 8'x8'). For a small extra charge, you can increase the foam density which will increase in the R factor. Check out
Factory Direct Spa Covers or
Poolandspa.com.
2) Buy a Thermal Cover. It won't work wonders, but every bit of heat retention helps. They're cheap, and some online sites may throw it in for free when you purchase a hard cover. It doesn't really matter where you buy it from, they're all the same. Basically it's just a heavy duty version of bubble-wrap. Keep the bubble side up. They only last a couple of years, then the plastic tends to break down.
3) Don't keep the spa at full temp (100-102) unless you plan on using it two or more times a week. This will save you the most $$ on your electric bill. Lower it to around 80 degrees (or put it in econo mode). Most spa's can increase the temp from 80 to 100 in about three or four hours (depending on how cold the outdoor temp is). This means you need to set it to full temp at dinner time, so it's ready to use later that evening.
Hope this helps. It worked for us.