Effective memes will be those that cause high fidelity, long-lasting memory. Memes may be successful at spreading largely because they are memorable rather than because they are important or useful. Wrong theories in science may spread simply because they are comprehensible and fit easilty with existing theories, and bad books may sell more copies because you can remember the title when you get to the bookshop -- though, of course, we do have strategies for overcoming these biases. An important task of memetics will be to integrate the psychology of memory with an understanding of memetic selection.
Some people argue that memes are not digital (Maynard Smith 1996) and that only digital systems can support evolution. Certainly genes are digital and certainly digital storage is far preferable to analogue. We all know that digital video- and audio-recordings look and sound better than their analogue predecessors; a digital system allows information to be stored and transmitted with far less loss of information even over noisy channels. However, there is no law that says that evolution has to be digitaly based -- the issue is really one of the quality of replication.
What, then, makes for a good quality replicator? Dawkins (1976) sums it up in three words -- fidelity, fecundity, and longevity. This means that a replicator has to be copied accurately, many copies must be made, and the copies must last a long time -- although there may be trade-offs between the three. Genes do well on all three counts, and being digital gives them high fidelity copying. So what about brains?