Dual n-back training will give you increased ability to hold things in short term memory but it's not something that can just be applied - it requires repeated practice.
There are numerous free applications you can find online. I've used a program called IQ-Mindware (
http://www.iqmindware.com) that I find very easy to use.
While these programs also claim to increase intelligence there are differing opinions as whether that's actually what's going on. As I understand it, regular use of dual n-back training increases you score on IQ tests but the main question then becomes whether that makes you more intelligent or just better at taking the tests :).
On the marijuana issue, I smoked on a daily basis for a number of years and can from personal experience tell you that it really reduces thinking, reasoning, memory and analysis capabilities. The good news is that you can gain it all back by not using and by having a good diet and lifestyle.