What factors should I be weary of when negotiating with external investors wanting to invest in my business?

Our business is seeking its first-round of external investment in order to expand. Like many business founders, I am weary of relinquishing shares and control to investors. I have heard a few horror stories recently about investors negatively impacting on businesses they have invested in. With that in mind, can you tell me what positive/negative experiences you have had with investors? More specifically, what areas should I look out for when negotiating investment?