As many helpers as poss (particularly for meeting and greeting, food distribution and saying goodbye/handing out party bags - always chaotic!)
and perhaps one extra helper available to "assist" or shadow any very shy or disruptive children. 6 yr olds probably old enough not to escape through front door but good to keep an eye!
Always take parents' mob phone numbers (in case of emergencies) and check about allergies
Don't offer tea or coffee to visiting parents as too much faff when you are busy with children (but maybe have something easy like a soft drink available if you want to offer something)
Winter party = have extra rack in hall for hanging coats/scarves etc or helpers put them all on a bed upstairs to bring down later.
Decide what you are going to do about presents - open once everyone has arrived - or quickly label each one with post-it note and put in basket for later (so you can identify/thank who it came from).
Organise two activities specifically for arrival time and leaving time ie so children can filter in or out at different times and join in (colouring/sticking large mural, simple crafts) leave more organised games for middle of party
Food boxes like these make lunch/tea easier to serve and much easier/quicker to clear up (dc sat on picnic rugs on floor?)
Mini juice boxes or cups with straws through lids are more practical than open cups (less spillage!) or alternatively mini bottles of water with names on.
Have available: scissors, kitchen roll, matches, bin bags, sellotape, plasters, blu-tack, (and bucket and Dettol just in case but hopefully not needed!) + 
for later 
Have rough timetable up on wall that you and helpers can follow roughly ie 2-2.30 arrival activity, 2.30- 3.00 game to break the ice (musical chairs?), 3.00 to 3.30pm pin the tail on the donkey or equiv then break for tea (allow 20 mins or so + singing happy birthday/cutting cake) followed by sit down game (pass the parcel) etc etc to finish by roughly x o'clock so that you can keep things moving in right direction and finish up more or less on time but be prepared to add in or take out a game depending on how it is going.
Second suggestion about having two parcels going around circle for pass the parcel or divide guests in to boys and girls and have one parcel each (and prep parcels in plenty of time/day or two before - they take ages to wrap!)
Second suggestion of extra small presents/sweets on hand for siblings who arrive at collection time
You may not have time to organise/want to do this, but we had magician for dd's 6th birthday party and it went down a treat!
6 yr olds quite like jumping up and down on sheets of bubble wrap (large bubbles) but be prepared for horrific noise - "bubble wrap bounce" makes a good energetic finale 
Sorry that was a bit of an essay - but I am a bit of a control freak when it comes to parties - find they go better that way. (Always had a bit of free play thrown in but for short periods of time). And I found that the more prepared I was, the less stressful/more relaxed it was on the day ifyswim.
Good luck!