In terms of $(a)$ the inductive effect and $(b)$ steric factors, account for the decreasing stability of $\mathrm{R}^{+}$:
(a) Compared to $\mathrm{H}, \mathrm{R}$ has an electron-releasing inductive effect. Replacing $\mathrm{H}$ 's on the positive $\mathrm{C}$ by $\mathrm{CH}_3$ 's disperses the positive charge and thereby stabilizes $\mathrm{R}^{+}$.
(b) Steric acceleration also contributes to this order of $\mathrm{R}^{+}$stability. Some steric strain of the three $\mathrm{Me}$ 's in $\mathrm{Me}_3 \mathrm{C}-\mathrm{Br}$ separated by a $109^{\circ}$ angle $\left(s p^3\right)$ is relieved upon going to a $120^{\circ}$ separation in $\mathrm{R}^{+}$with a $\mathrm{C}$ using $s p^2$ hybrid orbitals.
Carbocations have been prepared as long-lived species by the reaction $\mathrm{RF}+\mathrm{SbF}_5 \rightarrow \mathrm{R}^{+}+\mathrm{SbF}_6^{-}, \mathrm{SbF}_5, \mathrm{a}$ covalent liquid, is called a superacid because it is a stronger Lewis acid than $\mathrm{R}^{+}$.