I think that one really important step to take when attempting to achieve a flawless base is to get the right moisturiser, day and night, for your skin. I think that this plays a key roll in how your skin looks because it is the first product to be applied onto clean skin both on a morning and on an evening, and if you use a moisturiser that isn't correct for your skin type, chances are it will start to effect what your skin looks like. My skin, for example, has a combination skin type. So I  need an oil free moisturiser, that isn't too heavy and preferably has some sort of SPF. After many trials and tribulations I have discovered that Benefit's Triple Performing Facial Emulsion works the best for me and my skin type.
This is one thing that I can't stress enough - ALWAYS take off your make up before going to sleep! Throughout the day our faces can get really dirty, so even if you don't wear make up, if you want to make your base as flawless as it can be, washing your face before bed will really help! Personally I use a cleanser and toner before applying my moisturiser both before bed and when I wake up. I have really been enjoying products from the Nars Skincare range, which I wrote a review on here.
Touching your face as little as possible will really aid in providing a more flawless looking base. Not only will it keep your make up looking fresh but it will also help to protect your face from break outs. So, don't pick your spots, don't rub your face, don't sit with your head in your hands or lean your face on your hand - they are full germs! We touch everything with your hands, so no matter if you've just cleaned them, avoid putting them on your face if you want to make your complexion as flawless as it can be!
This may just be down to personal experience but I have used a number of foundations over the years, and my skin seems to me to have looked its best, and most flawless, when I have used a more premium base product. I am not saying that all high-street foundations are amazing and that all luxury foundations are great, just that you need to carefully pick and choose which foundations work the best with your skin. For me it has been Benefit's Hello Oxygen Wow and Nars Sheer Glow, both of which are on the premium side of things. That's not to say that one day I may stumble across a high-street foundation with a compatible formula for my skin type that also provides a flawless look for me.

What are your top tips for a flawless base?