Productivity can be a very tricky thing to gauge so it is always a good idea to pick a standard measurement that you can compare against to track your productivity. With that out of the way, there are many different things that can affect productivity as @Atrophius mentions but I'd like to focus on a few different aspects that can help your teams productivity.
Good Source Control - All projects are built on top of the bedrock of the source control being used. Make sure your developers know how to use your source control properly and make sure that you are using a system that allows your developers to work how they need to. Its also important that your developers "check in early, check in often" to help solve integration issues as soon as possible and when they are smaller problems.
Automation - One of the best things you can do for your developers is work out a powerful and flexible automation system. The most important things to automate are your builds, tests, and releases. If your developers have to remember to integrate some code and run a bunch of tests manually, its never going to get done. With solid project automation, your developers know when a build breaks sooner than later, they have the most current documentation, and they know their projects tests are helping maintain the quality standards.
Testing Testing Testing - If your developers aren't writing tests regularly and often, they should. By writing a comprehensive set of tests as you write code, your developers help keep the system design in mind and also ensure a good level of quality to their code. These tests should be automated and should be run with every code commit.
Time-Boxed Development - Its always a drain on a developer if a feature or project just drag on and on and on. The best way to combat this is to pick very small, specific goals that can be achieved within about a 2-4 week time period. Once your team has decided on these goals, do everything you can to avoid interruptions and have these features done, done, done at the end of that 2-4 week iteration. By done, done, done, I mean it is unit tested, automated, documented, and in a releasable state.
Better teamwork and communication - I find that the teams I have been on are much more productive if we are effectively communicating with each other as a team. It is important to build the communication channels and trust between developers so that they can come to rely on each other and work through problems faster and with better results. I've found that using daily Scrums we can communicate with each other quickly and effectively.
These are only a few suggestiong, but I find that without these main areas covered, your productivity gains by adding monitors or chairs isn't nearly as useful.