WELL JUST SAY NO! and it is fine you may think. However my vision concerning that is a bit different, I always see what we can do and if the percentage is high I ended up accepting the tasks. The outcome of that is spending many hours of your life doing different things in parallel which is you are not able to concentrate into one single task.
TIME COSTING and YOUR RESOURCE WASTED with poor performance. Wondering If I said no on that time.
My lessons learned here was accomodate your time schedule first before saying yes. If does not fit your schedule, you have to have the courage and braveness to say NO!
However, my colleagues also gave me good advises such as negotiating and exchange tasks with others to make it fit the time schedule....
I know it is very basic working skills however it is difficult and also needs to have the habit to train your mind everyday to that. Balancing the personal achievements and the achievements of your team or company in your mind is very hard task...