Common mistake: Getting too much gear. The number of bodies in a Harlequin army is small enough as it is, and overburdoning the characters with equipment is not helping either.
Second: Setting characters up to act alone and unsupported. This just gets them killed fast. Apart from the DJ's, all other Harlequins work best together.
Third: Not suiting to your enemy. Who are you fighting? As a general list goes, this will "equally weak" against any opponent, but if you can see who you are playing, and what his army is, you can make your Harlequins better suited to fight that threat.
Shadow seer:shuriken pistol and bio-ex ammo, harlie kiss, power blades, d-field, phase field
Total:141pts
Drop ammo and phase-field. This guy should stick with the troupe anyway, so he does not need to be zipping off on his own with the phase field.
Solitaire:Neuro-disrupter, harlie kiss, d-field, phase field, power blades.
Total:175pts
If you are not Fleet of Footing this guy you are losing inches and extra attacks. Drop the neuro-disruptor and get a shuripist. I would suggest dropping the phase-field as well, but if you want to use him as a one-shot cruise-missile into some enemy heavy weapon-squad you should stick with the field. Then you should also get a mask.
3 DJs:1 bright lance, 1 missile launcher, 1 shurieker cannon
1 DJ:shurieker cannon
Total213pts
Go for two brightlances and a ML. Or 3 BL. You need that kind of firepower.
4 mimes+1 mimic:1 harlie kiss, 1 power weapon. Mimic has power blades, bio-ex ammo
Total:168pts
Drop the ammo. Fleet of Foot the entire unit to cover and flank the harlies, and cover the Solitaire before he assault.
9 troupes+1 leader:1 power weapon, 1 harlie kiss, 2 plasma pistols. Leader has power blades and bio-ex ammo
Total:303pts
I would suggest dropping the two pistols, as they are horribly expensive, along with the bio-explosives. Get a kiss for the leader instead, or split the squad into two, remove the leader and add a few more bodies into the squad. With the support of the ShadowSeer you will be able to move them up the field quite easily.
Good Luck!