How to get a FPGA design that will definitely work on actual hardware
At a place I worked there were two camps of FPGA designers. One camp I called simulate, simulate, simulate or s cubed. The other camp was all about design.

The s cubed guys used a simulator like modelsim, they would come up with an initial design via coding methods and\or blocks in the design suite. Then they would simulate it and find the things that wouldn't work, then change the code. This process was iterated several times until they came up with a design that worked.

The design camp (which I preferred) would design the waveform on paper (or digital paper like visio), exactly what was required. Then come up with a logic diagram. This is a self-documenting process. Then the diagram was translated to code (the code and the diagram were 1:1 if there was something in the diagram, there was a process for it in the code). Then it was simulated, and the simulation waveform was compared with the designed waveform on paper, and was expected to be the same.

I ended up doing both, sometimes I would get into s cubed mode, and it wasn't very fun. I found that I lost sight of my goal sometimes. For example, I would change a state in a state machine, and the change would ripple down to the next state, then I would have to fix that. I ended up spending more time than thinking about it. 

Which camp would you rather be in? I think there needs to be rigorous design, do what works for you, but I think the more detailed and rigorous you are at designing, the less problems you will have in the long run. I gave some examples of what is possible, they may not fit the organizational structure of your workplace. The reason why design detail and careful planning is so useful, is it forces you to think about what your doing. It makes it easy to debug. Develop a design workflow that allows this to happen.
Also, get really familiar with the simulation tools and write good testbenches that will test all of the conditions that the simulated device might experience. This of course needs to be balanced with time. For example write ADC HDL code that will simulate the device in your simulations.

The most valuable tool to have in FPGA design (in my opinion) is a good testing procedure that will allow you to fully test your design and run it through its paces. An FPGA design cannot be expected "to just work" it takes effort to make sure all of the pieces work. If you spot errors, then go back to the simulation and design and learn what the differences between a simulated FPGA and RTL are. That mainly comes with experience, but if the design works in simulation but not in hardware then you need to find out why there is a difference.

A few key things that I learned:
1) Sanitize your inputs, the clock and reset circuits need to be clean or you can get metastablity propagating through your system. Know what a dual rank synchronizer is. There are many different topologies for reset circuits, know how to use them (there is a great article out there on the web, I don't have it on hand though).
2) Get the requirements of the design up front and then design around those. If the people around you won't give you definite requirements, then come up with some on your own.
3) Matlab fixed point toolbox is great for simulating control systems and DSP applications, but you might not have access to that. It's a great way to prove a design before you code.
4) Design comes first, then coding, then simulating.
5) Strongly typed, also keep signal names consistent on pcb schematic and hdl. (this is also why I much prefer VHDL over verilog.