There are no good passive braking methods for such speeds that don't smash the mission. The key factor is that maximum mission deceleration is limited to fairly low values, which means a large braking distance.
Solar sails. Using the notation of HDE-226868's answer, the braking force is $F\simeq10^{-5}\frac{N\cdot a.u.^2}{m^2}A/D^2$. It is obvious that mission deceleration will be greatest at the minimum distance: $Mg_{max}\simeq10^{-5}\frac{N\cdot a.u.^2}{m^2}A/D_{min}^2$, where $M$ is the mission mass and $V$ will be the mission velocity. The work of the braking force, braking from infinity, will equal the mission's kinetic energy at infinity:$MV^2\simeq10^{-5}\frac{N\cdot a.u.^2}{m^2}A/D_{min}$ (the term corresponding to maximum distance vanishes). Dividing these two equations, we can obtain the relation $g_{max}D_{min}\simeq V^2$. It holds for any solar sail regardless of efficiency, material, target star &c. For interstellar but not relativistic speeds, let $V=\beta c$, then $D_{min}\simeq\beta^2\frac{c^2}{g}\frac{g}{g_{max}}\simeq10^5\,a.u.\beta^2\frac{g}{g_{max}}$ where $g$ is Earth gravity. Substituting this back into the mission deceleration equation, we see that the area density of the sail is inversely proportional to the fourth power of mission velocity: $M/A\simeq10^{-16}\,kg\,m^{-2}\frac{g_{max}}{g}\beta^{-4}$. If the mission is carrying something squishy like humans, $g_{max}$ must be on the order of $g$. Assuming a mission velocity marginally attainable with known technology $\beta=0.1$, our braking sail must weigh at most $10^{-12}\,kg\,m^{-2}$. This is quite beyond the capacity of any known reflective material. (For comparison, a one-atom-thick sheet of aluminium would be around $10^{-7}\,kg\,m^{-2}$.) A fully solid-state probe could, perhaps, sustain decelerations as high as $1000g$, but the sail would need to sustain them as well. Solar sails aren't likely to be useful for braking an interstellar mission.
Braking on interplanetary medium. Unless one is traveling to a young planetary system full of dust (which, ipso facto, will have no useful planets), IPM seems to be only dense enough to be a nuisance and many orders of magnitude away from being useful for braking a mission traveling at interstellar velocity. If we assume that IPM material simply accretes to the mission, the braking force $Mg_{max}=\rho SV^2$ where $\rho$ is IPM density and $S$ is the cross-section of the mission vehicle. Mean IPM density in the vicinity of Earth is on the order of $10^{-19}\,kg\,m^{-3}$[1], and it falls off with distance from the primary as $r^{-1.3}$[2]. For given $V$ and $g_{max}$, mission vehicle must have $M/S\lesssim\rho V^2/g_{max}$. The latter fraction is just the braking distance: $D_{brake}\sim V^2/g_{max}$, so $M/S\lesssim\rho D_{brake}$. Using again $g_{max}=g$ and $\beta=0.1$, $D_{brake}\sim10^{14}m$ and $M/S\lesssim10^{-6}\,kg\,m^{-2}$. This is much less flimsy than the solar sail, above, needs to be. However, the braking distance ($600\,a.u.$ in our case) is much larger than the effective radius within which IPM is sufficiently dense.
If hypervelocity impacts of IPM particles produce explosions and eject material, the braking force seems to be enhanced by a factor of $\sim\frac{v_e}{L}V$, where $v_e$ is the exhaust velocity of ejected material and $L$ is the specific heat of sublimation of the materials involved. For $\beta=0.1$ and $v_e\sim 3\,km\,s^{-1}$ characteristic of chemical explosions, this factor is on the order $10^2$ and the mission can be heavier, $M/S\lesssim10^{-4}\,kg\,m^{-2}$ (though it must now withstand the explosions, a daunting prospect). Nevertheless, this does not remove the tyranny of braking distance and probably precludes the use of this braking method.
Electrodynamic braking on interplanetary magnetic field does not appear to be effective [3].
