Guernsey and Jersey are the 2 biggest islands of the group and are serviced by daily ferries to France (St-Malo) and England (Poole summer only, Weymouth and Portsmouth) as well as by air. Alderney and Sark are independent islands within the Bailiwick of Guernsey, Alderney is reachable by air from the other islands or from England and by passenger ferry during the summer only from France. Sark on the other hand is only accessible by passenger ferry from either Guernsey or Jersey and sometimes from France during the summer.
There are no cars on Sark so there is absolutely no means to hitch this exclusively pedestrianized little island, the ferry is though pretty expensive so someone might be willing to try (if possible) to hitch a boat ride with a fisherman. Hitching the ferry wouldn't be a solution as they only carry foot passenger and makes their money out of it. Daily return from Guernsey is about 22 pounds (by end 2007).
Alderney is the northernmost and most remote island of the group, it is only serviced by seasonal foot passenger ferry services from France or by air. So hitching to/out of the island is quite an impossible task. The island is also pretty tiny (5km long x 2.5km large) so walking around it would be no problem whatsoever, but there are a few cars on the island.
Herm is the smallest island of the Channel Islands group. Herm is a property of the States of Guernsey and only a handful inhabited the island which is linked by daily foot passenger ferry to St. Peter-Port - Guernsey. Cars and bicycles are banned from Herm, so hitching is only reduced to assessing the opportunity of getting a ride to the island with a fisherman.