Birthright and Superman Returns to some extent give us alot of plausible excuses
Superman wears tights and lifts cars, Clark wears baggy/ill fitting clothes and has trouble with peanut butter jars.
Superman is outgoing and 'visible' Clark is shy and blends into crowds.
Superman is an ideal alpha-male, while Clark plays has a crush on Lois, She's so focussed on attaining the Ideal she over looks it when he 'chases' her.
Superman has deep blue eyes, Clark wears 'coke-bottle' glasses that difuse their color.
Superman is confident, Clark fumbles and stumbles over his words.
In SR both Richard and Jason started to see a resemblence, Jason because CK was in front of a TV playing Superman's work, and Richard saw the basic similarity and did the math on their respective absences, and convenient returns, when CK heard him putting it together and plant the nugget of the idea in Lois, he did something very 'un-super' and Richard shook the idea off as coincidences
By appearing as Superman in bold primary colors and w/o a mask who would suspect he spends time as any one else. In 'Death and Life' upon first meeting the JLA, he ponders..."How do they find the time to be in the JLA and still lead a normal life, if they even do, as far as the world knows I'm Superman full time."
for a real life example, look at the 'Heroes' character of Mr. Bennett/HRG, when the actor appeared on a talk show I didn't recognise him until they started talking about show details.
The key to the 'glasses as disguise' is that they need to be the thick framed old school style, not the wire thin metal frames that are fashionable today