If the sole objective of the trip is a trek in the Tassili, flying in is the most convenient way.
Before the 'troubles' began, Air Algerie operated a convenient direct flight from Paris to Djanet. Though there were persistent rumors
that they will recommence, this was squashed with the kidnappings of foreign tourists in 2003. For the time being the only viable way is taking a connection via Algiers. Fortunately Air Algerie have rescheduled their
domestic flights to Djanet and Tam to connect with the european arrivals and departures, so it's possible to avoid having to spend a night in Algiers both ways. The flights are pricey, a
roundtrip from Europe will cost 600 - 750 Euros per person, more if one of the few European airlines are used to fly till Algiers (typically a domestic add on to an international ticket will be much cheaper
than buying the two tickets separately.) With Air Algerie it is possible to obtain a ticket flying in to Djanet and returning from Tamanrasset, or vice versa, for the same price.
There used to be occasional direct charters to Djanet from Paris, Munich, Zurich and Milan, however most of these stopped in 2003. As confidence (and security) slowly rebuilds, it may be worth while to ask around. These charters are usually taken up by groups organised by large tour operators, but frequently they have a few free places left which can be bought by independent travelers.
The flight itself can be one of the highlights of the trip, especially at morning and late afternoon, when the low light contrasts the huge dune ranges of the Grand Erg Oriental. Arriving in Djanet is a wonderful experience, with the little airport composed of nothing but a runway in the sand and two low buildings. The luggage is offloaded from the airplane into the sand on the edge of the apron, to be packed on to the waiting camels and Landcruisers. A culture shock indeed...
This used to be the mode of choice for most independent travelers to the region, however due to the tigtened regulations following the 2003 kidnappings (mandatory guide, no off piste driving), this means have become quite a hassle. Obviously if the Tassili is included in a larger trip (including visits to the more remote outlying areas of the Tassili to the North & South of Djanet) this is the (only) practical way. However if the Tassili is the only main purpose, one needs to weigh the benefits and disadvantages: The trip via Tunisia is a long and mostly boring one, with really interesting landscape starting only when reaching the northern
fringes of the Tassili. It is not possible to take a car up to the plateau itself, so it must be left behind in Djanet. Overall the cost in both direct expenditures and time may well be more than taking a flight.
Chris Scott's Sahara Overland plus the various sahara travel websites have plenty of information and advice on how to organise such a trip.