I have to do "casual Fridays" at work a lot, and have similar issues. I wear my slim dark jeans (I have a navy and a black pair) or leggings a lot. I often put a top and jacket/long cardi over it (a la Nowrinkles above) and use scarves a LOT. I have a few long tunics too, that go mid thigh, worn over the leggings - jersey ones that generally are gathered under bust and then fall loose from there, occasionally I'll even wear one of the longer ones over thick tights with heels.
A pair of knee high boots (flat soled) can help make that look too.
If possible, try to bring a similar colour down through your whole body, it can elongate and slim you down. So a navy jacket over navy jeans - top can be white or a colour under that, and scarf can be different too but the jacket will bring the top and bottom together. Or use a scarf similar colour to your legs to allow different coloured top altogether.
An interesting brooch on a jacket (a big bold one) or nice jewellery can draw the eye towards areas that you feel better about too.
And walk tall - if you FEEL confident and LOOK confident, you'll be amazed at the difference in perceptions (I had an event a while ago when I'd actually put on over a stone - but dressed comfortably and nicely for it, gave myself a talking to before I went in, and got loads of compliments about all the weight I'd lost and how well I was looking!!! )
Take your time to do your hair and makeup, so that you FEEL good about yourself, it WILL make a difference.