How can I stop my spectacles from sliding down my nose all the time?

Adjusted. All spectacle can be adjusted to create a better fit. Doing this yourself, however, will probably result in a broken frame. See an optician.
Glasses. Choose a frame that is not too heavy. Make sure the skin of your nose is clean, and not greasy. Select a high index glass if you have a strong prescription so the lenses will be thinner and lighter.
Select and adjust. Selection of frame for bridge style, weight of lenses and proper temple length with accurate adjustment makes for a comfortable pair of glasses. Heavy, poorly fitted lenses and frames are uncomfortable and slide down the nose bridge.