Okay, so let's say you have a really big goal. You want to take out that oWoD Ventrue Primogen by eroding his power base until the Daeva Prince gets so agitated he executes or banishes him. No easy task. If you start by hurling baseless accusations around, or take a step wrong, you'll likely end up the one executed. So what do you do? How do you take control in this situation?
Well, if you've been keeping accurate notes you'll ideally have a base to work from about both individuals' likes, dislikes, allies, enemies, assets and what-not that you can target, undermine, emphasise or whatever. If not, don't despair. You can start picking up this information. But what do you do, exactly? How do you find it out? What do you do once you know?
Since each situation is different I can't advise you on just what you should do but I can advise you on how you can figure it out. Other than going back over your notes, you can draw up a mindmap (basically a series of related details linked by lines to show you how they spring from each other) that can often jog your memory and help you brainstorm other options. You can learn more about this technique in trusty Wikipedia.
Another option is to simply write down your goal, brainstorm options, choose the best of those options and write them down on separate pieces of paper and then list out more options, ideas, etc. until you've whittled it down to baseline goals (with a record of how those little goals connect to the larger goals, thus ensuring that it may well lead to your primary goal rather becoming random tangents).
You can also list out your various skills (anything you have a dot in), alliances, and assets, and brainstorm ways they could be used to achieve your goal. Who knows? That dot you have in Socialize could come in handy seducing their childe into telling you information while drunk on drugged blood at the bar.
If you and the other player characters are on the same side, it's a good idea to try and get them involved in the brainstorming and planning stages but don't let them take over if it's your main goal. They have their own skills and knowledge to bring to the table and are more likely to be motivated to really give it their all if they feel like they're a part of it. You can even get them to do the same methods that you are.
One thing I'd suggest is that you jot down all suggested plans (a plan often has parameters, multiple minor goals, and a visible marker of hoped for achievement) and then get everyone to try this exercise. So that everyone gets their time to talk about the situation, tell them all to be quiet and have the person who came with it explain the plan. Then give each player (or player character) a minute or more to talk (uninterrupted) about their thoughts on a particular plan (say Plan A). Do the same with each person, ensuring that no one is interrupted, and the plan-maker doesn't comment on what's been said. Then let the plan-maker have their final say, fixing up any misunderstandings, and commenting on what's been said. Do this with each viable plan. And yeah, it may seem a bit artificial and forced, but it could work for your particular group.
So there you have it. Some ideas on how to use written plans and party input. Let me know if you have any hints, tips or tactics on these sorts of methods. Especially let me know if you are going to try any of them out.