The ideal friend is turning into a salt insoluble and crystallize

You should do this reaction using dry acetone as a solvent. Excess of methyl iodide will evaporate and rest quaternary ammonium salt will crystallise as solid. Methyl iodide will have to add 2-3 times during the course of reaction.

You could try mixing toluene solutions of both reactants. Use azeotropically dried toluene.

You may try a neat reaction using microwave irradiation in sealed reactor.

Initially the energy uptake will be small, but with increasing ion formation it will heat up rapidly, thus you can use even microwave "inert" solvents. Crystallize the salt from appropiate solvent (DCM, toluene) and consider sublimation (againd Cardice cooling finger) to obtain really pure material.

P.S. it is commercial:

and has been used as electrolyte in:

Electrolytic solutions and electric double-layer capacitors using them

Jpn. Kokai Tokkyo Koho (2012), 13pp. CODEN:JKXXAF; JP2012109539

Synthesis in acetone, 3 h, 30 C.

http://worldwide.espacenet.com/publicationDetails/biblio?FT=D&date=20120607&DB=EPODOC&locale=en_EP&CC=JP&NR=2012109539A&KC=A&ND=5#