I tried to do it many times before, because Robert Bruce was recommending to do it at least 10 minutes a day but it was always difficult for me and only after I had begun practicing transcendental meditation I understood that it is mostly all about mindset... Now I do it much easier. I stand in the position hug a tree and mentally repeat a mantra over and over again (instead of mantra you can count if you wish) and after 5-10 minutes I fall into the altered state of mind and at this very moment all tensions go away and I can easily stay 30 minutes without any difficulties.... It is all about mindset.