Credit card is great, especially if you stay at a lot of Marriotts, like most of us here (5x). I don't use my credit card much besides Marriotts and restaurants, but I can hardly tell much of a difference at 2x the points for the restaurants in points accumulation
There are a few ways to sprint to get credits or points, but after a while, it gets exhausting to just chase points, especially since most of us here work full time, are on the road a lot, and would rather just spend time with our families than looking for points here and there. In other words, I go for the low lying fruit, but that's it.
-so there's some websites you can use to get more points
-use the credit card like it's going out of style--every $3000 or so you get a free night credit (not to actually stay a night but to count toward elite status)
-figure out which hotels will give you points on using the restaurant. (I'm not sure if you actually have to be staying there to get full points or not)
-referring a friend is a great way, but I never feel like I actually see those points.
-transfer points. You can get somebody to get the credit card then have up to 50,000 points transferred a year. (I've never done this either)
-buy points (I don't do this)
-family trips with stays at Marriott (this is what I do a lot of)
Some hotels will offer a bonus 3000-5000 just for staying there. Those are good ones.
-wash rinse repeat for years.
If you are going for a level like gold or platinum, it's pretty easy to spend a focused year on getting it. It's year 2 and 3 that get exhausting. I'm guessing most people who get a level challenge aren't able to maintain their status past the first year.
(That's me and Hilton right now).
Aside from staying at Marriott properties the next best thing is to make use of the Chase Marriott Premier Credit Card. I charge everything that I can to this card and then pay the balance in full each month. This card allows you to earn points with every purchase and elite night credits for every 3K spent.
If you have Lifetime Status and don't need the Elite night Credits I recommend putting all of your spending into the Ritz Credit Card. With the Ritz card you earn points at the same rate as the Marriott Card but get a 10% Bonus on your points at the end of each calendar year.
Chase also has a couple of other credit cards that allow you to turn Chase Ultimate Rewards Points into Marriott Points or Airline Points.
Occasionally some hotels will offer Bonus Points for a stay. Several times I've earned 2,500 or 5,000 Bonus points for a weekend stay at the Fairview Park Marriott in Falls Church, VA.
Lastly there is the online shopping portal option. Shop My Way allows you to earn extra Marriott Points for online purchases made through their shopping portal.