I think Smart Nappies would technically be classed as prefolds and wraps. They are quite like Flips but I wouldn't reuse the outers as often as I would with a Flip because there is like a hammocky bit for the insert/pad to go in that can get quite a lot of wee on it. Usually, I sniff it and see!!! Sounds gross I know! If they aren't dirty, I usually leave the outers to air after use (so have them on rotation, rather than reusing it straight away - it lets them dry in between uses and it's easier to have a new one ready to use straight away). When it's time to do the next change, I check that the outer still smells fresh. If it doesn't, I put it in the wash and get a new one out. With the Smart Nappies, I found they tended to smell of wee after only two uses or so. You can wipe them out with a damp cloth between uses if you like, to get more uses out of them, but I think it's all the nooks and crannies in them collect the smell!
If it's stayed clean I just wipe it over, let it dry and use again 
