The False Emperor's realm is too vast, its armies too numerous to actually hope one could establish a permanent foothold in Imperial space. At best, the Black Crusade could hold the Imperial hordes at bay for some time, but ultimately its momentum would be exhausted, its supplies spent and the enemy would just keep coming.
Also, one need to consider the very nature of the forces united under the banner of Crusade in question. There are four broad categories of troops Chaos warlords could muster:
1. The Lost and The Damned. They are numerous and differ wildly in training, equipment and devotion. They form the bulk of the Chaos armies. Their numbers can easily be replenished in the field, as one's Black Crusade takes new territories; ignorant natives, prisoners of war, criminals, pirates and seccessionts - all and any of them could be press-ganged into service with little effort. But the truth is, the Imperium could just as easily recruit ten times as many sacrificial fodder (known as the Imperial Guard). So the battlefield role of the Lost and the Damned is not to triumph over the Imperial hordes but to lock them in a fight long enough to allow the true warriors of the Black Crusade to meet their objectives.
2. Daemon Engines. Warp-powered and / or daemon-possessed warmachines constructed by the Dark Mechanicus; Defilers, Slaughterers, Blight Drones, countless other infernal contraptions, and - greatest of all - Chaos Titans. They are a lethal weapon when used wisely, but one should always remember their numbers are finite and cannot be replaced in the field. Construction of a daemon engine is a complex process that can only be done in a daemon forge, which are usually located deep in the Chaos-controlled territory (Eye of Terror, Maelstrom, Screaming Vortex, etc).
3. Daemons. Denizens of the Warp, who are powerful and deadly foes, but only available for a limited duration of time. The main weakness of the daemons is their link to the Warp, which grows weaker over time and must be reinforced on a regular basis for them to remain in the material realm. So, when relying on daemons, one should either stay close to the warp rift, from which they would draw strength, or find some means to create such warp rifts. For example, Word Bearers Space Marines know the secret of creating daemon worlds, or - if one could not win their support for the cause - one could raise a cadre of sorcerers who shall perform the necessary blood rites... In the end however it shall only delay the inevitable, as sooner or later one should simply run out of people to sacrifice, or the daemons - gorged on the blood and souls - would go out of control and embark on an orgy of death of their own making.
4. Chaos Space Marines. Champions of Chaos, elite warriors who are the most deadly weapon in one's arsenal. Their strengths are legendary and don't need to be advertised. But so are their weaknesses. Infighting between various warbands alone is bad enough, but then every warband also has its own agenda and objectives it will strive to achieve, often at the exclusion of everything else. At the beginning of the Black Crusade Chaos Space Marines are all united by the same goal - to break through the Imperial lines. Once that goal is achieved however, the old rivalries and matters of personal gain rise to the fore. Truly great warlords could control the warbands, but only for a degree and for a time.
With the above-stated kept in mind, one should understand that any Black Crusade - no matter how successful in its opening days - is ultimately a short-lived endeavor. Should one tarry in the Imperial space for too long, one's forces would dissipate - either due to losses or to the infighting and (especially if things would start looking grim) to the outright desertion - and the Crusade would end in the shame of defeat, its objective not met. So two things should always be kept in mind when launching a Black Crusade:
1. Avoid protracted engagements at all costs. If the Imperial forces succeed in dragging one's armies down into the war of attrition, then one is lost. That means that one should plan his Black Crusade in great detail and prepare some means of bypassing the Imperial bastions guarding the edges of Chaos territories. These means could be some great weapons of mass destruction, like the infamous Blackstone Fortresses unleashed at Cadian Gate by the Warmaster Abaddon, or more subtle developments such as the Plague of Unbelief created by Herald of Nurgle (Typhus) or the Nexus Arrangement used by Dark Apostle Marduk at Boros Gate. If one lacks access to such extraordinary means, he must rely on brute force and momentum of the attack, throwing one's forces into the meat grinder in the hope to break through the bastion worlds and into the Imperial space. Once the way is breached, one should cast away any unreliable forces, allowing them free rein. These shall be hunted down and eventually crushed by the Imperials, but with their deaths they shall buy precious time for the Black Crusade to achieve its true objectives.
2. The true objective of the Black Crusade should be not conquest, but destruction. It takes a handful of weeks to despoil a planet, and a thousand years to rebuild it. And if the invaders are thorough in their work, there shall be nothing left to rebuild. Once the intended target (or targets) is wiped off the face of the galaxy, one should immediately regroup his surviving forces and head back into the Chaos territory, whether it be the one the Black Crusade was launched from, or another. In one's wake shall remain burning wreckage of some Imperial world and - if one is truly wise - hundreds of thousands of cultists and demagogues scattered across dozens of star systems. And so with the Imperial resources spent on repairing the damage done to the Corpse-God's realm, the next Black Crusade should be able to strike further into the enemy territory, turning yet another heathen world into a slaughterhouse. And the cycle shall repeat itself until the day when Terra shall burn and the False Emperor be cast down from it's rotten throne.