Ok here's what I did and it helped a lot.
Everyone already knows to smack their preferences down to low, reduce draw distance, turn off property/owner and ban lines.
Having done all that and still getting crippling lag at times I went hunting for lesser known and less obvious solutions.
What you may not know is the normally hidden Advanced menu has some thingies you can fiddle with to further contribute to a less laggy SL experience.
So, first if you don't see an "advanced" tab next to 'file, edit, view, world, help' etc along your top bar, press CONTROL ALT D to bring it up (some users may also need to use SHIFT).
Then... Avatar Rendering Cost, avatars with lower ARC figures are less work for your computer. Under Advanced > Rendering > Info Displays select Avatar Rendering Cost. Note that it will significantly reduce your FPS if you leave this switched on, so only use it when you need it.
Next enable multi-threading if you have more than one CPU or more than one core, if your computer is a dual core or quad core for example you can offload some of the work onto another core, splitting the load.
Under Advanced > Rendering select Run Multiple Threads. This hands off texture decoding to another processor core. Users with only one core/CPU will likely see no improvement. This option might increase the incidence of viewer lock-ups, but this seems to hardly ever happen.
Running multiple threads significantly reduced my personal lag.
Finally if you use FRAPS you can keep an eye on your framerate, alternatively you can use the SL meter. Press SHIFT CONTROL 1 (that's the number one on your keyboard). A small display will pop up on the right. This is the statistics display and is a little more detailed in its analysis then the standard SL lag meter (the one with the 3 lights, green, yellow, red."
The Advanced menu was designed for people wishing to test code and fiddle with the open sourcy nature of SL but it comes in handy for us regular mortals too.
Obviously if you're not comfortable with delving around the guts of the app, then don't.
But the above tips do help quite a bit.
Props to massivly dot com for those Advanced menu tips.
Next, Turn OFF voice chat. Despite propaganda and other nonsense, every "feature" added to the system increases lag. Voice is not nessacery somewhere lik Erie, so turn it off and that's one less feature your system (and the client) has to cope with.
Lag could be YOUR fault... how many prims does your avatar comprise of including clothing and accessories?
A lot for some (I'm guilty of this sometimes), flexi, sculpted and tortured prims all pile on the lag, plus badly scripted items such as that free gun you got on xstreet can all contribute.
If you clock in fully dressed and armed at a five hundred prims plus scripts imagine what you plus all the other players are doing to the sim you're currently in.
Lag is not always the sims fault. Do your part client side to reduce your personal lag and at least you can say "It's not me!"
Later peeps.