right.
things that have meant we have survived flights 
make sure you and dh are clear on what jobs you are each doing at the airport. It will be stressful enough anyway, and trying to answer questions while hanging on to stimming/over excited child is not easy.
I tend to leave dh to do all the check in stuff, and literally sit down at the foot of the check in desk and read a story to the dds. yes, we get gawped at. but hey, it works for us.
Trunkis are great - the dds have one each. one for toys/books/wipes/change of clothes and one for food (both are multiple food intolerant, and you can never book special meals for children - even if you manage it, it's not somehting they are likely to eat! - so we have to take all our own food. if you have this problem too, I have furhter tips for you). The trunkis give children somewhere to sit other than the floor, and can be used as entertainment while waiting to board the flight, without unpacking all the toys you brought. also an extra toy at destination - always a bonus.
you cannot beat an ipod touch or ipad for battery life and entertainment value - music, films, apps in whatever form you like - worth its weight in gold. and, if using an itouch, easy to pack, and doesn't weigh a tonne.
a good magazine or two (for the children!) also useful - usually have a load of different htings to do, with stickers, colouring etc. I found some brilliant ones in Sainsbury's I think last year - they were mosaic-by-numbers sticker books. kept dd2 quiet for a good hour.
for airport familiarisation - I thik it is Manchester airport that has released an ASD familiarisation guide - amberlight has linked ot it before. There is an Usbourne book - a big chunky one, wiht a wind-up aeroplane, which describes everything that happens in the airport, security etc, and you run the plane around the tracks on each page. might be useful for talking things thorugh beforehand this is it
if you need ot take extra liquids through - a particular cup, or because of a certain juice etc, this is possible. you need a letter from your doctor stating it is a need (we have to take dd1's juice with us - she cannot drink apple or orange, and will not drink water. on a 10hr+ flight, this is a problem. we have had exemption form the 100ml rule because of this)
is your 18 month old still small enough to go into the car seat extra that straps onto the bulkhead table bit? we used ot bung dd2 in there, with something new and shiney/exciting (she spent the whole flight to California clutching a new squirty bath toy in the shape of a dinosaur when she was about 16 months old!) - leaves your hands free if so.