Depending on the department, the rules regarding the payment of conference trips can change. You should ask the secretary about how to proceed.
In general, the university does not accept that you buy flight tickets through the internet. You should use a travel agent for that. In case you choose your own agent, you may have to pay from your own money and get the reimbursement later. Note that reimbursements are slow procedures (sometimes up to few months). If you use accredited travel agencies, like Ophir Tours in Givat Ram, the payment can be done directly to them.
In case you need money in cash for the trip, take the completed form to the Finance Department at the Levy Building. They can arrange you will receive the desired amount of money in your office (they provide different currencies). In case you do not need it beforehand, you can fill in the form on your return. In any case you will have to provide receipts of the expenses (including boarding pass).
If you need to print a poster, there are graphic services at the Sherman Administration Building. They also print posters in cloth, so you can fold it. Be aware that sometimes they ask more than 24h to print it. In case you want to print a poster in paper in one hour, you can use Graphos, at Ben Yehuda 33. They sell tubes for carrying the poster.
To avoid spending too much time on security check at the airport, when you travel for business, you can ask to be already included in the security system. You should check in your department who is the responsible for it. Once you fill in a form, this person will include you in the system and give you back a document with a bar code, which should be printed and handled to the person that first checks you before checking-in. This procedure can also help in case you have troubles with travelling with your laptop or external HD.
Nesher Taxis operate a shuttle taxi service between the Jerusalem area and the Airport for a fixed price per person (slightly more than the bus fare). Call a day or so in advance, give your address and the time of your flight, and they will tell you what time to be ready. They pick you up at your door. If you live at the dorms and they ask you to come to the front gate, insist that you want them to come to the dorms, as it is a long walk. Call 02-6257227. Avoid calling on Saturday hours (call before).