If you're strapped for cash, the best thing I can tell you to do is stay away from the things that require major outlays of money. I got my little 200 acres of heaven back in 2001. In the beginning, I could not find anyone to do my food plots for me. As a result, I used those methods that required little or no expense.
Here is the website the wildlife biologist showed me:
As a for instance, he showed me some spots in the hardwoods where the understory was being choked from cedars. He recommended pushing over the cedars or cutting them part way to create more light on the forest floor and also provide cover for deer. Shortly after his visit, we had a series of ice storms and wind storms and a bunch of the cedars in one part of the property were knocked over. This became THE deer bedding area on the place. Another thing he suggested was "edge feathering"-- basically letting the fence lines grow up. Where I have done this, the deer and turkey have benefited greatly.
Probably the biggest improvement I can tell you to make is simply get down and scout the property and become intimate with it. I won't kid you, ponds and food plots and all that are great ideas. However, on 400 acres you probably already have everything already in place to keep and hold a good herd including a few monsters. The trick is just scouting it out and finding where the best places are to hunt the deer. It took me over a year to really acquire enough of a feel for the place to accurately predict where to find a decent deer on Opening Day, and another year to stop second-guessing myself and settle down into a rhythm.
By the way, I used to be big on water sources as a key to deer hunting. Of course, a lot of the books I'd read were written by guys in Texas, where it is semi-arid. I found out that in our part of the world, deer can usually get their water from puddles and off vegetation and spend very little time in the Fall around water. Even last year, with a severe drought, there were hardly any tracks around my stock ponds.