a PR 6 requires about 6x the amount of link juice - so if you have a PR5, do 6x what you did... but you should really just forget about PR.
PR 4 buddy PR 4 I had three pr 5 sites at one stage two are now PR4 and one remains at PR 5, yes been there done that. The only way to support it's up keep and survive Google's updates is hope to get some PR 6 sites linking in oneway, either that or about 30,000 plus min - quality pr3-5+ type links to rise to PR6. Also hate to burst your bubble PR 4 today is what PR 5 was about a year ago I think the big 'G' has raised the bar on their ranking so going to PR 6 today would by like striving for a 7 rank a year ago , daunting task but if your the person for the job, good luck! ROOFIS
By just doing basic SEO tactics and gaining backlinks, your PR will rise naturally. Don't do things to try and gain PR, Google hates that. Just bring in traffic from as many outlets as you can and your PR will naturally rise over time
A PR5.. wow!!! You need to keep building backlinks to stay at that or build more and more backlinks to increase it.
If you want to gain PR6 then you must post your link on PR6 and above websites and blogs. Link exchange with other PR6 website or more will help.