Only Accept Products You Want
Yes it's nice to be offered free stuff, but whats the point in taking something that you're not going to use or isn't relevant to your life or blog. It annoys PR practitioners as they're looking for coverage and it takes the opportunity away from other bloggers who could actually want to review it.
You're Not Obliged To Review
Products are gifted to you a blogger in the hope that you will write about them on your blog or social media but you are under no obligation to write about it. Unless you have signed a contract or made an agreement that you will definitely write about a product, there is no ethical or legal basis for being forced to write about it.
You Don't Have To Write A Good Review
Just because you're given a product for free does not mean that you have to write a good review. Brands want honesty from their customers, they and your readers won't appreciate being lied to and being told this is the best mascara ever if you actually found it clumpy and irritating. Again you don't have to write the review, if I have a bad review I also try to include a few good points such as attractive packaging or great customer service.
Consider Your Blogs Integrity
If you're known for being the blogger who has a specific style or opinionon- say only wearing heels- people will notice and question your morals if you suddenly write sponsored posts about how much you love trainers and that you "live in them". I always consider my audience and my own integrity before taking on a sponsored or gifted post simply for a pay day or free stuff (no matter how tempting it is!).
You Must Disclose
The Advertising Standards code can be very confusing regarding bloggers, but this quote puts it simply "a blogger who is given money to promote a product or service has to ensure readers are aware they’re being advertised to." not doing so means you risk losing your site or having any search terms regarding your site removed from the internet. No only is it legally wrong but you are also lying to your readers by pretending that this isn't a biased post. Even if its just an asterisk in your post title or a quick sentence at the end of your post, you must do it. My post Why You Must Disclose Sponsored Posts explains it all.
I hope this has been of some help to you all