When it comes to coding web apps I've learned that a three step process works best for me.
Take some time to really understand the problem you are trying to solve and the potential solution.
Once you think you know the right way to go, go for it. Don't dilly-dally. Jump in and hack your solution together. Remember you've thought through it already so don't waste any more time.
Don't second guess yourself. Have confidence in what you've done. If it is wrong or if it breaks something - don't stress - you will fix it.
Progress is thwarted by fear and over-thinking. Don't be hasty or careless. Think, act and believe.