Keeping a deck of cards always at hand, and a Half Dollar in my pocket (that's why jeans manufacturer's give you that little pocket in the main right hand pocket - to keep your magic coins separate

)
I practice all the time - and just in case I have an idea for a sleight or routine, I usually make sure there's always a deck in the glovebox - as I have been known to run out halfway through a restaurant meal and grab the cards to test out a new idea!
My ex-fiancee was not really very tolerant in the latter half of the relashionship - as I used to sit practicing riffle shuffles and stuff while she was trying to watch telly... but these days I just do it as and when.
My worst habit is trying to do coin rolls and stuff at work, where I have a glass topped desk... and halfdollars tend to be quite loud when they land on it!
I'm also dubious now about practicing in traffic jams, as the new phone law as of Dec 1, 2003 dictates you can't handle a mobile phone in the car... I wonder if there's a law about practicing your second-deal???
You can never practice too much!
For serious practice sessions, I tend to sit at the kitchen table - usually alone, with my mirrors (oooh errrrrrrrrrrrrrrrr....). I can sit for hours on end - and a tinny or two of beer and some Walker's Sensations with some black olives usually go down well.
You should make time and set it aside from disturbance to practice - especially when trying to learn a new routine. It needs to sink in.
That said, it is also beneficial to practice in front of trusted family/friends, to check angles and flashes etc.
But as pointed out, only do this when you're competent enough to keep the item in your hands!!!