For increasing the PR of your site you must get high backlinks from high PR websites and blogs. Well, lets take the example of DP – if you are really serious about getting good PR for your site – then i suggest you that you post daily at DP (but don’t spam) – post useful information – (just think while posting whether it could be of any help to anyone) – and this way you can increase the posts – and don’t forget to place your blog / website link in the signature space so that ur link will be displayed on every page where you have posted – this is really a good way of getting decent PR.