NaOH to stabilize aqueous NaCN
Hydrogen cyanide is an acid with a pKa of 9.2, meaning that at pH 9.2 in water you have about a 50-50 mix of cyanide anion and hydrogen cyanide. If you used sodium hydroxide to adjust the pH of a NaCN solution to, say, 12.2, you would inhibit the decomposition of NaCN by shifting the equilibrium to 99.9 percent cyanide anion and 0.1% hydrogen cyanide. Each pH unit higher gives you an additional factor of 10 lower concentration of HCN.

Another factor is temperature: since HCN boils at 28 C, it tends to evaporate from a room-temperature solution of NaCN, driving the reaction to the right. The solution would be more stable if kept cold.

Finally, it would be better to keep the solution in a tightly closed bottle, since carbon dioxide could be absorbed from the air, acidifying the solution and releasing hydrogen cyanide.