When it comes to design, my biggest piece of advice would be to keep it interesting. Avoid buying furniture sets and matchy, matchy pieces. This goes for decor items as well. Keep it all visually interesting by mixing it up.
Try combining framed art with objects in a gallery wall. In an all art gallery wall use different mat sizes, frame sizes, shapes and colors. If you're decorating a mantle or bookshelf, stack books to add height and then display smaller trinkets on top.
I love the look of flat rectangular books with a round ceramic jar on top. Or a piece of square art on a bookshelf with a round mirror layered behind. Our furniture tends to have a more streamlined square look so add different shapes in the surrounding furniture. Use a hexagonal side table on a round rug in your living room. Keep it visually interesting.
Woods, ceramics, metals. Velvets, linens, grasscloths. Mix it all. You can also mix wood styles and colors. This can help provide a "collected over time" look and give your room more dimension.
This is where you can really have some fun. It doesn't matter if it's a ceramic vase from Asia or a basket from Peru, they can work together in the same space. My favorite change to a traditional piece of furniture is by adding a contemporary fabric to the upholstery. Or using contemporary chairs at a traditional dining table. It's unexpected and keeps old pieces looking fresh. Keep in mind that as a whole, the room should flow with a certain style. However, adding a few pieces of furniture or decor that challenge that style can really give the room a polished look.