I have been travelling to Thassos independently since 1985 and believe me it is a lot easier now than it was then. In those days you had to fly to Thessaloniki and make your way to kavala either by taxi or bus, and on getting to kavala you would have to catch the flying dolphin to Thassos.
Now with the arrival of major tour companies it is easier, you can fly directly to Kavala airport on a chartered flight which you can usually find on the internet. ( These flights are available from May until Mid October) On arriving at Kavala airport it is easy to get a taxi to the ferry port of Keramoti, this will cost around 9 euro’s and takes about 10 minutes. The ferries run frequently and as a foot passenger it will cost you less than £1.00.
There are plenty of rooms available in Limenas (Thassos town) and the only time that it may be difficult to find one is between the last week of July and the first 2 weeks of August leading up to Panagia day. This year I was told that something like 72000 people arrived on the Island in the days leading up to Panagia day and some 20000 cars (Panagia day is around 15/16th of August I am not completely sure) not including foreign tourists. The rooms are basic but clean and most have there own shower expect to pay 20 euro’s a night for a room but this is per room not per person.
I have travelled to Thassos from the U.K in the Winter this involves getting a scheduled flight from Heathrow to Athens and catching a connecting flight to Kavala. Last time I flew with Olympic and it cost around £250 for the return flights and there was very little hanging around in Athens about 1 hour before you caught the onward flight.
Thassos is a great place and I have lost count of the amount of times I have visited Thassos go on do your self a favour and give it a go.