I can recommend Jim Dunlop delrin picks. They come in different thickness so experiment with a cuple to see which is best. I use .72mm on my cittern and mrsleveller uses the next thickness up on her mandolin. When I first started using them I was amazed at the improvement in tone. Only problem is, they go dull after about a week so I buy them 10 at a time (they're only around 25p each). You can get them online from Strings Direct.