The Site I like a lot and that got me started writing sponsored posts makes it so easy to find all of the information for your post and even includes a program where you can drag and drop the required links and pictures! It's Social Spark and I've been with them for almost a year and have done some great sponsored posts and learned a lot about great companies!
I love working with them because they have a simple Pay Pal payout that you can request after $50 in posts and the program guarantees that your points will be awarded! I also like that you can set your price per post, pick your advertisers and you even have a reasonable length of time to get your post written! The best feature though, is that you do all the work on their site before you publish, so that the advertiser can pre-approve your post and you are not having to go back and forth making changes on your blog! I also love that they have great disclaimers that all the advertisers understand will be included in the post - transparency is great!
Social Spark works with all sizes and types of blogs so it's totally worth registering, even if you're a smaller blog starting out!
Disclaimer - by registering through my link, I receive an affiliate bonus for directing you to Social Spark