??How to increase thrust??

Hi all,

I need to increase the amount of thrust I am getting from my
propeller.  I am limited in the fact that I do not want to change my
propeller or gear ratio.  So to my best knowledge, I have the
following options:

(1) The motor I am using is a Mabuchi HV motor.  I think I can switch
to a Mabuchi LV motor and get more thrust with less flying time?
http://www.toytx.com/1015vmic.html

(2) I am currently using one Li-Poly battery (4.2V, 135mAh) to power
my RFFS-100 receiver and motor.  I think I could hook up *two* Li-Poly
batteries in series to increase the current to my motor?

(3) Any additional options?

I am just wondering if both of these are feasible?  If so, which one
would you all recommend?

Thanks in advance,
-Bill