Assuming all you want to do is change the advancement rate (not magic level, or anything else), it's pretty easy.
1) Determine the advancement rate you want (say, twice as long to level).
2) Multiply XP awards appropriately to achieve the rate desired (so, multiply the awards by 50% to get the rate given above).
3) Multiply the treasure awards by the same amount. However, you then need to add a bit more treasure (for a reduced rate of advancement), or take a bit off (for a faster rate of advancement). This is because the game assumes that the party will burn some disposable items, and that these should be replaced. If the advancement rate is slowed, they'll burn more disposables, so need a touch more treasure.
The biggest problem comes with NPC gear. You need to change the value of this gear to the PCs, without reducing the amount of useable gear that the NPCs actually have (as doing so can damage the CR system). The solution here is to adjust the resale value of acquired gear - most items an NPC has are going to be less good than the items the PCs have, so most of them are going to be resold. So, if you reduce the resale value of found items accordingly, you reduce NPC treasure values without changing the gear the NPC has. (I'm not sure what the exact value should be, but it's probably close to the same multiplier you've used in steps 2 and 3.)
4) Keep an eye on character progression. If the party are gaining too little treasure, throw in a chest of gold at some later point. If they're gaining too much, cut back for a while. If need be, you can adjust the resale value for found goods - either have a war push up demand (and therefore the price), or a recession which prevents people from splashing out on luxury items (like resold magic items).