Taking credit at work

First, try to say to him something such as "Oh, I can see you've been working on the idea we discussed yesterday". If it doesn't help, try to keep "a written sign" on your work such as "I hope that this project will fulfill the expectations" with your signature. And if this doesn't help, have a friendly talk with your boss telling him that you are afraid that your contribution is not fully known. 

I know some people which are faced with such situations. :( If you work on a bigger project for a week ...for example , give your boss everyday a feedback of what you have done up to that moment, so that he can see that you are working on that project, or tell  him what your feelings are about the work you are doing: if you enjoy it, if you think you would finish in before the deadline...things like that, so he can see you are the one who is doing the work.If this doesn't work, try something harder...

 Also see this:

http://www.cvtips.com/back_stabbers_at_work.html

There are two issues here:  that your work is recognized, and that your co-worker can't be trusted.

So the obvious ones are to label your work thoroughly (even in electronic files, embed your information (for instance go to file, then properties, and begin entering in everything you can with your name, the project, etc)

Also sell and self promote these with your manager, and where appropriate, ask your manager to review your work before you submit.

Be warned though: do not put your manager what they perceive as being in the dispute to 2 coworkers, as that never works.  Just be confident and keep moving forward being a positive competent professional.

The second and main issue is that a coworker can't be trusted.  They are unable to give credit, misrepresent the work they have done, and other scummy activitiy. 

So do not support their work anymore, and ask them what they hope to accomplish by doing their current activities.  This gives everyone a chance to save a bit of "face", but remember, this is just a soft way of notifying them that you are on to them.  After this, just stop helping this person. 

And get a different job away from scum.