You need to change a value in the "rule_values" table of your database, see the top (how to log in to your database) and bottom (the command to run) of this page:
http://www.eqemulator.net/wiki/wikka.php?wakka=How
Bigger numbers = faster XP gain. I believe 0.5 is the default, so 1 is double XP, 5 is 10x XP, etc.
Keep in mind zones have individual XP multipliers too. Some zones grant triple XP, some less than normal XP, and most are in between. If you set the rule_values XP modifier to something like 5 and then go to Bastion of Thunder (which has a 2.7 XP multiplier or somesuch) you'll level at absurd rates.
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