Remember that there is a 30 cent fee for every L$ buy so buy amounts that are "signficantly" larger than that.
I.e.:
$1 buys you L$182 (L$182/$1)
$5 buys you L$1,222 (L$244/$1)
$10 buys you L$2,522 (L$252/$1)
$25 buys you L$6,406 (L$256/$1)
It doesn't mean you should just buy a whole lot more than you need, but if you frequently find yourself buying only single dollar number's worth of L$ then it will make sense to buy a larger amount.
As an example: one L$ buy for $25 worth would give you L$6,406 but 5 $5 buys would give you L$6,110 or about L$300 less.
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You can get about 3.5% more as well by doing limit buys instead of the default market buy.
Short summary:
- Linden Exchange / Manage / L$ Exchange Settings (
https://secondlife.com/my/lindex/currency_settings.php) - set to Advanced... you only need to do this once
- Linden Exchange / Manage / Increase Credit to put a US$ balance on your account ($25 minumum)
- Linden Exchange / Manage / Buy L$ (do not use the Buy L$ that's left of Manage since that one will only let you do market buys)
- look at "Open Buy Orders at the Best 20 Rates" on the right side and in general you'll want to go one number *below* the top listed rate which right now would be L$268/$1
- put the numbers in the "Buy at a Specific Rate (Limit Buy)" box and buy
Or more practical: $25 on a limit buy order would get you L$6619 which is L$213 more than you'd get the way you're buying L$ now.
(Feel free to ask more questions

. I just did a very short summary since a lot of people don't feel the 3.5% extra is worth all the hassle so I just mentioned the practical steps rather than why or what exactly you're doing in each one)