Statistically blended families last an even short time period than original families. Therefore it is imperative to deal with conflicts as soon as they arise.
Keep communications open with your new spouse and with the children. Communication is the key to dealing with conflicts. Be open and honest about how you are feeling without being accusing.
Do not let little things pile up. Tackle them as they arise. Remember that he may have been a bachelor for the last 5 years and it didn't matter if he put the lid on the toothpaste or left it off. If it bothers you so much, accept that it obviously doesn't bother him and put the lid on yourself and be done with it. No amount of whining is going to help him remember. You can mention it once offhandedly and let him know that you found the lid for the toothpaste and put it back on. If that doesn't spur him into the action of replacing the lid when he is done, then forget it. Why make a big deal out of it? It is one of those little endearing things that we ladies learn to live with.
On the other hand, if he is still in communication with all his former leading ladies and does not have children with them, it is your right to let him know that it bothers you and is not normal behavior.
Pick your battles. You can live with the toothpaste lid, but you may not want to live with all the former leading ladies.
Don't let the little things that bother you get out of control and become a huge dinosaur in the living room. Make a time and place to discuss conflicts and get them out in the open as soon as possible to prevent hurt feelings and resentments from building up. The longer you stew in your resentments the larger they become. Its kind of like walking around the dinosaur in the living room. No one really wants to mention it and upset it, but someone really needs to get it out of the way.
If the conflict is marital, do not include the children in the discussion. Some things are meant to be private between a husband and wife. Keep them that way. Otherwise you will be getting all sorts of conflicting advice from the children and possibly even the ex spouses. Not to mention having the children start siding with one or the other of you.
When conflicts arise regarding the children, make a united front. If you need to talk to your spouse in private get the facts and tell the child you will discuss it and return. Have them wait there and go into another room. When you return present a untied front. If children think they can split you up by causing a conflict, many will do so.
State the consequences and make sure that the child understands that it is coming from both of you, not just the stepparent. Presenting a united front will show the children that you are making every effort to make this relationship last.
Recognize that not all conflicts will have an agreeable resolution. There are simply times where we have to agree to disagree. In these cases, state your case and acknowledge that you understand their side of it. Then as a mature couple agree to disagree and go on with life. Its too short to waste on a meaningless conflict that 20 years from now won't even matter.