As any oil painter will tell you, there's a hundred and one ways to keep your oil paint brushes clean during and after a painting session. Everyone seems to do it slightly differently, so here's how I do it!
Some days, if I know that I'm going to be painting all day or even again the following day, I don't clean them at all. I leave them dipped in linseed or walnut oil which coats the oil paint and appears to slow the oxidation enough to keep the paint useable.
Rembrandt did this using a tray that you can see in the studio display in The Rembrandt House Museum.
Here's some images of my process I sent to someone experimenting with oil paint - they were using White Spirit to wash their brushes out all day, not a great idea.
Please don't use Turpentine/White Spirits or any type of odourless Spirits indoors for cleaning your brushes all day unless you have excellent ventilation or you're painting en plein air.
The odourless mineral spirits OMS, although smelling more pleasant still pose a risk to your health. Here is an example safety data sheet.
Here's a better way to do it, not as good as a Rembrandt tray but good enough for me:
Have as many brushes available as you can, I don't mean hundreds but having a plentiful supply negates the need to clean them out so much during the painting day.
Keep some Linseed or Walnut oil specifically for keeping your brushes clean. Yes, it's expensive but remember this is the oil that is used in your oil paint and what you are doing is essentially diluting and removing the paint in your brush with more oil.
Mix your paint with a palette knife and not your brush. That way there's less chance of working the paint up into the ferrule. Of course, your painting style might involve loading the brush completely so feel free to skip this!
When you're finished with that brush for the time being. If it's not the end of the day - you might need to pop out for a couple of hours, just dip the paint on the brush in the oil and leave it on the side. I have a little pot of oil for this - see below. When you want to use the brush again, wipe off the excess oil and carry on painting.
If it is the end of the day wipe out the excess paint in a rag. Dip the brush into the Linseed oil careful to not use too much, you only need to cover the paint. The oil will work itself up into the brush anyway. Dab the brush on your palette to remove some of the diluted point. You can repeat this a couple of times but try not to use too much of the oil - again, it's expensive!
Its the end of the day (or night!) so you can get the brush ready for the next session by using an ordinary bar of soap and water - a white bar of soap shows up the pigment better. Keep the bristles as in shape as possible but you still want the bubbles to get right up into the bristles to bring out all the oil and pigment. I use my hand (non-toxic paint only!) to massage the brush until the bubbles are completely devoid of colour.
Wash all the soap out with water and shape the brush. Ideally you can hang it upside down to dry, some people wrap a small piece of card around the bristles to reshape it whilst drying but I've never bothered, I just leave them flat on a table to dry until the next day. 
There is, of course special brush cleaner and conditioner, that you could look into but this method works for me. I'm always interested to hear how other people look after their brushes so feel free to drop me a line :)