Use shared keybinds
Binding the same keys for two different specs has been useful since day one, but it becomes even more important when players are switching specs all the time. Rogues have the luxury of having a relatively small number of abilities to bind, and most of them are cooldowns or other utility abilities.
The basic structure of using combo point generators and finishers allows us to make playing two different specs nearly identical in terms of bindings. For your assassination build, put Mutilate and Envenom on two keys, and simply reuse those keys for Sinister Strike and Eviscerate in your combat spec. Because action bar switching is built-in to the dual spec mechanic, there's no reason to not use identical bindings.
Reactive abilities like Feint and Kick should always be bound to shared keys in all of your specs. You need to be able to activate them instantly and without hesitation, so using the same key trains your muscle memory for when you need a quick reaction time. Secondary abilities like Recuperate and Sprint should also be bound to identical keys, if only because it makes them easy to remember. If you're looking for guidance in binding your keys, I have previously shared my tips on this topic.
Customize your addons for each spec
Many of today's popular addons are "dual spec-aware," and so they can tell which one of your dual specs is active. Each spec has its own things to remember.
You can customize your addons like Power Auras to only show specific auras when you're in a particular spec. I have my 35% life Backstab warning set up to only display when I'm using my assassination build, while my Revealing Strike reminder only pops up when I'm combat. Other addons like EventHorizon will automatically adjust themselves based on your spec and will only display bars and timers for abilities that you actually have available at the time.
Choosing the right gear modifications
Unfortunately for us, combat and assassination have very different stat valuations. It's not really possible to gear for both at the same time without weakening both. Assassination rogues are required to reach the spell hit cap as quickly as possible and then start stacking mastery. Combat rogues favor reaching the expertise cap instead, followed by tons of haste. Mastery is one of combat's worst stats, beating only critical strike rating and the white hit cap. Expertise is in the same boat for assassination, which puts the specs at odds when it comes to gems, enchants, and reforging.
What you can do is abuse agility's potency to minimize the amount of stat contention between your specs. By using primarily agility gems in your gear (except for those needed to match agility socket bonuses), you can eliminate the need to re-gem your gear when switching specs. Most of your enchants will be biased towards agility, with most slots only having one viable option, anyway. You can even choose to enchant your gloves with Greater Mastery, which is good for both specs, allowing you to keep all of your enchants the same.
Addons soothe your reforging pain
Reforging your gear, on the other hand, is going to be necessary to play both specs at their peak performance. An amazing rogue recommended Reforgerade and ReforgeSaver to me, and now I'm passing them on to you. You simply use any of the compatible reforge calculators (WoW Reforge, korner's Complete Rogue Reforger or bsoft's Java tool) and dump the output into Reforgerade, which will do the enchants for you. You can then follow that up by using ReforgeSaver to memorize all of your reforging sets, allowing you to maintain as many reforging profiles as you like. While it's not going to reduce the cost of reforging your gear, it does cut down the amount of time and effort you spend by a significant margin.
Practice, practice, practice
While having the ability to switch builds on the fly gives you a great deal of flexibility, you actually need to be able to play both specs for it to be valuable. I specifically mix up my builds when running dungeons or raid bosses that we have on farm, as it allows me to stay sharp on both of my specs. The only way to really learn the small nuances that define a build is to practice. There's no replacement for experience, and when your guild asks for you to swap to a particular build for a fight, you want to be ready to answer that call.
Check back every Wednesday for the latest rogue strategies, from rogue basics and kicking your interrupts into high gear to how to handle your dual-spec rogue and how to pickpocket top tips from top-performing rogues.