Humbrol do something called “Decalfix”, but this method works for me:
first of all, mask off the cockpit canopy with either Tamiya masking tape (and a scalpel/craft knife. Do NOT be tempted to use a Stanley knife, if you want to keep your fingertips**…) OR Humbrol Maskol. This is a purple rubber solution – like thinned down rubber glue – but beware, it eats brushes. I use a cotton bud.
Then give it all a thin wash of GLOSS varnish (Revell Acrylic #01) before you do the decals, and allow to dry. apply the decals, then another coat of gloss. Then a coat of matt (Revell Acrylic #02) for military types, not for civil types.
If you don’t have access to an airbrush, you can get aerosol cans of the gloss and matt varnishes, but be careful not to mix brands, as some of them have been known to bubble & blister. If you know you’re going to be doing a number of modern jets, try get the Tamiya Aerosol colours (TS32 Haze Grey or TS48 Gun Grey); same if you’re going to be doing a lot of WWII types, Dark Green, Dark Earth, Slate Grey, “Sky” for British types; Olive Drab or aluminium for US types, and so on…
If you have undercarriage & undercarriage doors, leave them off until the very end. Same with underwing pylons & weapons
** ask me how I know…