This is a question a lot of people seem to want to know. Not only for the purposes of getting stronger in deadlifts, but in any exercise they wish to get stronger in. It is not a simple answer, nor is it a cookie cutter template that can be applied with everyone. So I will only answer by giving general tips to follow as some might apply to some and others it doesn't.
Key to gaining stronger is pushing yourself, getting more reps, using more weight, eating more then what your body burns, supplementing properly and the list goes on. Simply just applying these will not increase strength gains overnight. All these concepts and ideas must be incorporated into a planed training program and diet regimen. A quality training routine that focuses on increasing strength on a weekly basis will do wonders compared to a training plan with no goal or substance put into it.
Rest-Pause sets also help in getting more reps. Rest-Pause sets is a method where you perform reps with a weight you can handle for 12 reps. Once completed, you rack the weight, rest for a couple of seconds, then un-rack and begin completing as many reps as you can. Then repeat the process for 2 or more times. This allows you to get in more reps with weight that you handle for let say maybe 20 reps. This is a very useful technique to employ when attempting to increase strength.
Another way to increase strength is utilizing food intake. This is often the most overlooked factor as everyone is focused on just adding weight plates or focusing on the training aspect of it. Food is essential and will produce wonders when the right combination of carbohydrates, proteins and fats are added in the right ratios. Supplements also help to a certain point. If your diet is crap or simply sucks, no amount of supplements will help you out. Supplements are there to help "supplement" the vitamins, minerals other important nutrients that you would not achieve through diet alone. If your thinking of spending $100 on the latest hydrolyzed whey, latest creatine or best pre-workout formula and your diet is complete and utter crap! Save yourself the trouble and buy a book on nutrition or use the internet for information.
There is a couple of other ways to get stronger, but these are going down to the vary basics. Whether you achieve getting stronger on deadlifts, bench press or squats, it all comes down to how motivated and dedicated you are about making that goal possible.