The first tip would be to realize that for a child, adjusting to a new parent takes time. It is not a one day process, to change the mindset and make you acceptable.
Communication holds the key, when it comes to step parenting. Talk to your spouse about the problems you are facing and also try to figure out the way out of them.
The best bet would be to be a friend to the step child, instead of trying to be a father or mother of the child. You need to realize the fact that you are a stepfather/stepmother and your bond will be different than that shared by father/mother and his/her biological child.
Another important tip would be to render a patient ear to the child. Chances are the more you listen to him/her, the better he/she would feel, and the faster he/she would accept you as a parent.
Insecurity is the major reason for the reluctance to accept the step parent. You need to reassure your stepchildren through your actions (and not words because the child is determined not to believe you!!) that you are not trying to steal his/her father/mother from him/her.
To build healthy relationship with the step child, one of the good options would be to take up a hobby with your stepchild. This way you would spend quality time with the child. Also, it would enhance the bond of love between the two of you, as the child would come out of the mental frame of you being typecast as evil or bad.
Follow the rules of the relationship - never speak negatively about the other parent. The child would then definitely think of you as a threat rather than a friend.
Be prepared to face a hard time with the kid. You need to have a lot of patience. Remember, it is not a one day job or there is no magic love potion that would make the child instantly love you and accept you.
Remember, you should never break their trust. If they have confided something to you, never let it out of your mouth. A one-on-one communication will help you in coming closer to them.
Lastly, give them your time and show interest in their daily activity. Right from how they are performing in school, to what they are doing after school, their hobbies at home, their nutrition, you need to know the tiniest detail of everything.