I used to clean my case about once a month, and was growing concerned that, as time passed, the fan was running for longer and longer periods, and with less demanding games.
After inspecting the airflow while running the system, found out that the culprit was the GPU fan, which I hadn't noticed because it was covered by a big lid. Removing it showed me several years worth of dust around the heat sink, which was blocking most of the airflow. A good cleanup, and things were working as great as day one again!
2. Remove the screws, and power cables, if needed.
3. Detach the card from the PC. You can see the plastic lid that hold it in place on the right side of the PCI-E bay
3. Extract the card and set it on a flat surface. This graphics card has a cover, let's remove it to clean inside.
4. Turn the card around and remove the screws on the borders. Now you can easily detach the cover.
5. This is how the graphics card looks without the cover
5. The fan can be separated to clean the disipator.
6. Put the cover back, and return it to the PC, by reattaching it, connecting the power cables, and screwing it back to the tower.
We're done! 50% temperature reduction for me (I was running at 72ºC before, with maxes of 86ºC):