SL will eat through 7Gb quite easily unfortunately

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A few tips are:
* bring your draw distance down as much as you can stand
* don't listen to streaming music (60Mb-ish/hour doesn't seem like much, but it adds up fast)
* don't watch streaming video at all
* don't clear your cache at every log-on (not something I like recommending, but just logging on and letting my inventory fetch takes 50Mb-ish download

hopefully your inventory is a bit leaner

)
Something you have less contol over:
* if you stay in the same spot, you'll use far less download than if you constantly tp from place to place
There's a few (free) monitor proggies that you can use to measure the amount of data passing through your puter. If you only have 7Gb/month I'd suggest downloading one and keeping a close eye on the numbers every day.
Personally for me, SL eats up about 500Mb-1.5Gb/transfer per day (just SL, no streaming audio/video even) depending on how long I'm online and what I do

.