It's easy to prep your parsnip bed and a great time to do it.  Parsnips, like their cousins the carrot, don't like terribly rich soil so don't add any manure or they will be stunted or 'forked'. They also need rock free soil as if they hit any rock while growing they will decide to head off in a bizarre direction and you won't get nice straight, large veg.  So ... 

• think about the bed you want to use (not freshly manured)
• give it a good dig over with a garden fork
• remove any rocks or stones
• leave the bed open for a couple of weeks - the birds will remove lots of pests for you and get a much needed meal too!
• once the soil is steadily around 50F or 10C, it's time to sow your seeds; put in as soon as this temperature is reached, parsnips need a long growing season
• remember to sow fresh parsnip seeds each year - they don't keep well
• sow seeds thinly around 1 inch deep in a row marked with a string
• be patient - they can take a couple of weeks to germinate!