GET GOOD ARMORED GEAR...Full-face helmet, chest/back protector, boots, knee/elbow armor, and gloves. That's not to say that it necessarily has to be expensive. You can buy used gear even, but I recommend new helmets. For example, I wear a $100 full-face helmet. It fits me well, has a DOT and SNELL rating, and has enough ventilation that I'm not sucking for wind. I also have inexpensive Bilt knee/shin and elbow/forearm armor. It protects better than the armor that came with my pants and jacket, but I got both sets for about $20 total new. My boots are $90 new Fly Racing Maverik ATVs. They're WAY more protective than the steel-toed hiking or work boots I wear for commuting, and my knee/shin armor tucks into them perfectly. You might find an even better deal on boots if you shop around. The ATV-height boot is about as low of a boot as I would recommend. My gloves all have armored knuckles, and I found my $50 Alpinestars Bionic chest/back protector greatly reduced on sale because it was last year's model. As hard as I have slammed my knuckles, shins, knees, elbows, chest, and back into things, the armor has more than paid for itself in avoiding any and all hospital bills.
If you can't attend a class, or ride with experienced buddies, you could try to find books or DVDs. Then practice the drills.
Knobby tires can help a lot. So can standing, as you found out. You can also try airing down your tires a bit. I also believe the Freewind has a smaller-diameter front rim and tire than the DR. A skinny 21" rim and tire tends to be easier to ride in soft stuff. It acts almost like a pizza cutter or an ice-skate. You may have to swap on a high fender to use a 21" rim.
A lot of DR riders also gear their bike down when riding a lot of dirt. A 15/42 sprocket set is stock in the US, while 15/41 is common in many other countries. 14/42 is a common choice for dirt, but the bike is still slab-capable. If you're not riding any slab, you could use a 14T front and go bigger than the 42T rear. 1 tooth difference in front is about equal to 3 teeth difference in the rear...14/42 = 15/45 = 16/48. I currently run 16/46, which is similar to 15/43 or 14/40. 14/42 and proper carb tuning lets the bike unweight the front end much easier.
So does re-springing and re-valving the suspension for riders heavier than anorexic Japanese teenagers.
More often than not, the throttle is your friend. There are a lot of situations where throttling is counter-intuitive, but it works. Stabbing the front brake is almost never the best solution, and I tend to use the rear more heavily when traction is questionable. I also often use it to stabilize the bike in slow maneuvers.