Change the molar ratios of substrates. i.e., 2:1 or 3:1 ratio of 3-pentanone and 3,4-xylidene and put the reaction directly in acetic acid at RT. Dissolve the xylidine in minimum qty of acetic acid and then add 3-pentanone. This method worked well for my schiff's bases. hope it will workout for u too. Good Luck. 

You can add molecular sieve or MgSO4 to the solution and toluene works better as solvent

I have tried with Molecular sieve but Molecular sieves do not work..Surely I will try with MgSO4

Thank you sir but how should I use it exactly? I mean during the reaction should I use?And in how much quantity?

use few drops of piperidine as catalyst

Try with 2-3 drops of Conc. HCl as a acid catalysed reaction. It worked for me when i was preparing Schiff bases with the ketones. It increased my yield. 

Ok. But i am adding glacial acetic acid.

Will it make any difference if I add conc.HCl

Instead of glacial acetic acid add Conc. HCl.

Conc.HCl should added to the methonlic/toulene solution of the reaction mixture.

OK.. This this time I will try with HCl..Thank you so much..

try in ethanol, i think u are using toluene.

No, Actually I am using benzene

But if I use Ethanol, how can I distinguish water and ethanol as they are miscible.

use dry ethanol an add magnesium sulphate to remove excess of water... :D

Try Solvent free synthesis by microwave irradiation

The microwave assisted syntheses of Schiff bases from different aldehyde and amines is carried out efficiently and high yield of product in short time has been obtained

May be you can useful from the following article