Here are a few ideas, might not be very good:
1. If you want a little brother, you have to RECRUIT one...
big/lil assignments done by "who recruited you the
most". Might not work in all cases.
2. The more members, the more $$$, the CHEAPER
your dues will be. Maybe even offer a dues reduction
for "recruiters" if possible (pyramid scheme??

)
OR, instead of the carrot use a stick: You didn't
do anything to recruit this week? $5 fine!!!
3. While recruiting, when possible, get the recruits to recruit.
Our chapter has used this successfully many times.
An independent is more likely to believe "Lambda Chi
is the best" when coming from another independent
(rushee) than an obviously-biased Lambda Chi.
4. Have the Delta organize all actives into recruiting
teams, and have teams deliver reports in meeting.
The "peer pressure" of "other teams are working,
ours is obviously a dud" can help motivate sometimes.
Could also do this on an individual basis....everyone
stands up and says what they did to recruit this week.
Oddly enough, the "formality" of "paperwork" (weekly
report) is enough to get some do do SOMETHING
instead of NOTHING.
5. Remind everyone of the rush theme: Make a friend
first, then a brother. Rushing doesn't have to be
anything formal, highly-organized or expensive
like a big party...just going to Burger King with a
Frosh can get a better brother than taking him
to a "knock out party". Guys recruited by
"knock out parties" are only interested in going
to "knock out parties", a reason many large fraternity
chapters turn into small ones in just a few years.
Not sure if any of these help, but I tried