Well...
1. Wait on the last day before the TOF-reset with reediming your daily mission rewards, do it after the reset and you have a headstart in exp.
2. Do the same with other activities which might give you experience, for example don't battle your oppenents immediately in your league, just wait for the TOF-reset.
3. There are some TOF-Leaderboards with experience rewards, they also count towards the next week.
4. As mentioned above the leagues are now a huge experience contributor, many of the top guys just spend Kobans to get the maximum experience out of the league. For example, last week there was only one Sexpert 1 league with 180 participants on Hentaiheroes, so if you wanted to fight everyone you had to invest Kobans, and because nearly everyone in this league was at a high level you also got a huge load of exp if you fought everyone and won against everyone in a week.
Let's say 179 oppenents with 3 possible fights each and 190 exp per win in average equals to 102030 experience... (Not very likely, since you need to win every fight but whatever)
5. Arena is also a small part in gaining some experience

There are maybe more things to consider but I think that explains it for the most part.