In theory, it's very easy: show up, buy a ticket for the public boat (Rp. 8000-10000 to any of the Gilis), and wait until the 20-25 people needed to fill up a boat show up. Early in the morning, this rarely takes over an hour, and you'll be told by the ticket sellers when it's time to go. Later in the day, though, you may be in for a long wait. If you aren't in the mood to sit around, you can also charter an entire boat for a fixed Rp. 185,000 to Trawangan, which is not unreasonable if you can share the cost with a few fellow travellers.
Life is made harder by the '''Bangsal Mafia''', a bunch of guys hanging around the port who will seriously hassle you to buy boat tickets to the Gilis from them, with various excuses like claiming that your ticket is no longer valid. Do not believe them and do not give in!
Warning* scams are plenty at this port. December 2008 I visited the port and was brought to what I thought was an office to buy a ticket. The guy had a book with a list of names and I was the 5th. After I gave him my money (1 US dollar) I was told to wait at the shady bar next door until the boat was filled. He said he would bring me my ticket there at the bar. I demanded the ticket at that moment or my money back. At that point things got bad and three other guys came in to take me to the bar to wait. Having a Lombok local with me (who was my ride from the airport) I held my ground till 10 long minutes later they brought me my ticket and I walked just 200m more to the end of the street. There I saw the large sign for ferry tickets and I saw the boat and just as I walked up they showed me a boat that was leaving.
Do not stop to talk to anyone until you see the water and the ferry ticket building! This is a scary place and it shakes everyone up that deals with it.
The way to sidestep this hassle is to book an organized tour. The largest operator, Perama [http://www.peramatour.com] arranges trips from Mataram and Senggigi direct to the Gilis for Rp. 100,000 per person, bypassing Bangsal entirely, while any tout or taxi driver can give you a lift and get a ferry ticket for around Rp. 300,000.
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