I agree with Bramblecot. As much mulch as possible once you have cut back the yellow ferns at the end of the year. Also, in the spring, a hefty sprinkling of salt. I know it sounds odd, but asparagus was originally a maritime plant and so tolerates salt very well. Weeds, on the other hand, do not like salt and nor do slugs.
Before you plant, make sure that every last bit of perennial weed has been removed. It is impossible to get rid of couch grass or bindweed once the asparagus has been planted. Also, you must plant the crowns in light, really well drained soil. If you have soggy or waterlogged land the asparagus will just rot. Really sad to see and also very expensive!