Good simplicity is a result of prioritization. The best simplifications are aggressively focused on the things people truly need…and the things that matter are guaranteed to be a subset of what your total customer base is asking for. The first step is to develop a realistic view of the customer base you have (or want) and what matters to them. Additional features don’t have to erode the simplicity of an experience, but compromises in either prioritization or implementation surely will.

The best simplification comes from an end-to-end redesign of the experience, though even then, complexity can creep back in through the iterations as requirements fill in. Once you’ve got the functionality you need, there’s plenty that can be done to simplify an interface by stripping it down one piece at a time.

Where to look

“Show, don’t tell.” Look for sentences and labels and figure out whether you really need them. If you see descriptions that would be better elaborated in dedicated help or FAQs, take ’em out and link to that instead. If it must be inline, could it be shown in an image or diagram more succinctly than words? If there’s text trying to get you to look somewhere else, maybe the options are in the wrong spot.

Scrub all terminology. If there are similar descriptions in different areas, pick the best and use it everywhere. Take the way your customers describe what they’re trying to do, and make the design reflect those terms. Once you’ve done these, get down to the smallest conceivable description for the current context (which may be just the name of an object, or a verb, if the context is obvious enough) Bonus: doing this will end up simplifying both translations and help documentation.

Look at options. Do they all need to be visible all the time? Progressive disclosure might be a better choice for dependent controls. Radio buttons can consolidate into listboxes. Sometimes controls could be combined, such as a checkbox to turn on a feature, plus an edit field inline to customize it.
Look for lines. Is there a lot of nesting, or divider lines? See if the sections are organized meaningfully, and then eliminate as much line noise as possible to keep those sections distinct. Play with text headers, solid-colored boxes, and/or indenting to see if those work better.

Look for icons. These can very quickly become a crutch for small spaces, if allowed to grow beyond a small set. Your interface might be in trouble if (1) there are more than 1-2 icons that have arrows in them, or (2) if the interface look like a bag of Skittles from afar. Evaluate where they’re used today and see if there are alternate representations for the behavior they embody (such as information, or action, or current context).

Look for colors. Once you’ve found them all, reduce your palette. Can you get away with variations on a single color? (or 2?) Be careful about using colors to designate tasks or areas of the product, as this system can collapse quickly if new tasks/areas are needed. Color carries connotation as well, so warm colors like red or orange will always seem urgent and should be used selectively.

Does it still make sense?

Once you’ve done all this, you can best gauge its efficacy by vetting your scrubbed designs with both new users and existing users. New users will be the best gauge as to whether it is understandable, especially in the context of other current features or applications. Existing users will help you discover whether you’ve overlooked any particular uses of the current design (which may reveal other needs to take into account), and hurdles they may encounter adjusting to a new design.

Keep in mind that no one likes re-learning something they already know how to use, so there will always be pressure to keep it as is. Concern is not an indication of failure. Preparing your teams for this in advance can make it easier to properly assess feedback as it comes in. Scrub thoughtfully, and your designs will be a clean slate for future innovation to come.