You can go to Lombok from Bali by fast boat. Different companies liaise daily Bali to the Gili islands (Gili Meno, Gili Air, Gili Trawangan) and Bali to Lombok (with often a stop in Gili Trawangan before reaching Lombok). Crossing will vary between 1:15 and 2:30 hours according to the boats and sailing conditions.
A new boat should liaise Bali (Padang Bay) to Senggigi in 1 hour sharp starting at the end of July 2013. Other destinations like the islands of Sumbawa or Flores are reachable sailing from Lombok but in one three days.
Lombok Fast Boats, one of the companies providing service between Bali and Lombok, seems to provide all security and promises a journey of about 1:30. A round trip from Bali (Padang Bai) to Lombok (Senggigi) in March cost around 100USD per person ( price listed on their website in March 2013). You can book online.
Ferry between Bali and Lombok
You can also reach Lombok by ferry from Bali: The most economic solution even if a more hazardous journey. Hundreds of Indonesian passengers use the ferry with or without their vehicles. It will take you a minimum of four hours in the best conditions to arrive in Lombok. The ferry leaves Bali Padang Bai (southeast) and arrives at Lembar Harbour Lombok (south-west). It is recommended to negotiate the price with porters before they put their hands on your luggage. There is little or no tourist organization in Lembar Harbour. So plan in advance your transfer between Lembar Harbour and your hotel to avoid paying for a too expensive taxi as distance car be quite long.
Also watch for pickpockets during your journey, specially during the rush when boarding or disembarking. And always keep an eye on your luggage at all times. In Lembar Harbour (Lombok) you may be offered “cheaper” tickets, but they might not valid. So check before buying. Better to get your tickets at the ferry counter or at one of the official agents on the Island.
There is also a ferry service from Lombok to Sumbawa Island (and back). Daily departures from Labuan Kayangan (East Lombok) to Pototano (West Sumbawa). Crossing in about 1:30.