1) LAYER: This was especially relevant to me in the winter, but I think it applies to any season. Having a handful of key pieces that you can layer, mix and match is a life saver. A few good pairs of maternity pants/jeans, lots of black leggings (I've been able to wear H&Ms black leggings in a Medium through my whole pregnancy) and some stretchy long camis are all things that have made pulling together outfits so much easier!
2) ACCESSORIZE: You can stay on trend and look cute with out having to invest in pricey trendy pieces from the maternity store. Keep your outfits simple and classic then jazz them up with fun scarves, statement jewelry or shoes. You are going to have to wear the same thing a lot (unless you have an endless clothing budget). Mixing up accessories is a great way to make it look like a completely different outfit.
3) THINK OUTSIDE THE BOX: Just because you are pregnant does not mean you have to shop at a maternity store. I only have 6 weeks left to go and the only things I've bought that are maternity are a pair of jeans, a few pairs of pants, a skirt and one sweater. Seriously places like H&M and Forever 21 are great for finding inexpensive pieces that you can buy a size up. Shirts and sweater come in much longer styles these days and they can cover pretty big bumps. I just bought a size M sweater at the Gap yesterday! H&M is great for basic pieces like stretchy dresses that grow with you. And remember that things like cardigans, blazers and jackets, that don't have to be buttoned, can be layered over basic pieces even if they are the normal size you wear.
4) ROCK THE BUMP: I have found that fitted clothes actually look better. Loose baggy clothes that are typically associated with pregnancy actually make you look way bigger than you are. Your bump is adorable so show it off. People are more likely to identify you as being preggo this way too rather than assuming you've hit the donuts a little too hard.
Here are some examples of how I've applied all these tips. Please exscuse the fact that they are kinda awful self-portraits!: