I only mix seperates when they differ completely in both colour and texture. For example, a navy suit jacket with a pair of grey wool suit trousers is a classic look. Equally, a grey herringbone suit jacket would look great with navy suit trousers. If you fancied something bolder, a burgundy suit jacket would work with all manner of trouser seperates.
Keep the colours different, but more importantly keep the materials and textures of each piece different.
Alternatively, get your arse to the shops and buy some sport jackets! =P