When you get to 15 daily boost gauge, don't turn in any more of the daily boost client orders until the 24 hour timer is about to expire, or the gauge has gone down by at least 1 (to 14.) Daily COs never expire when accepted. Daily Boost Gauge Client Orders will only refresh the timer on the daily boost gauge to 24 hours when it's at max 15/15.
If you aren't premium, keep only up to 10 of these. Otherwise, there is no other COs in the game that give tons of exp in comparison to meseta. The rare material guy in franca's cafe has COs that give 100,000 Meseta each, but gives almost no EXP if none at all. The Rare material guy's COs cannot be used to send Auxiliaries.
There's also a guide by @AnamanaAU in my pinned Guides and Help Mega-Thread that deals with Making 3,000,000 Meseta Weekly. You will be making alt characters on the same ship/server by the way. (Alts will need to reach 50+ before they start making tons of money/etc.)