There's no real secret. Just have Assists with a high Lux percentage, and be sure to be active during LUX up time. Combine that with hunting event heartless as those tend to give out good lux, and aiding your team in battling Raid Bosses and you'll gather a lot.
And one last tip. Raid Boss subspecies, signified by Red coloring rather then purple drop more LUX by default. Battle one with a decent Assist deck and during LT and you'll get a LOT of lux.
...I guess what i'm saying is strengthen your deck, and play strategically.
1) Assist cards. Higher LUX% means you will earn more.
2) Attack Power. Dealing more damage to the bosses will gain you more LUX.
3) Party Chain. After attacking the boss, if another party member attacks within 5 minutes there will be a chain. Chain provides bonus damage to the boss, earning you more LUX. To truly gain a significant chain damage, you need at least a chain of 20. So for weak parties, it is recommended to use 1BP hits till 20 chains are reached, then attack normally for the exponential damage. This strategy is still commonly used in active parties.
Edit: And of course, you earn double LUX during LUX Up Time.