Somebody in Sweden did his Master's Thesis on "track-before-detect":
Interesting because it's really just a well-known statistical technique, but of course difficult because you'd have to do it in real-time.
In short, if you use a single scan to decide whether or not a target is present, then you have to decide on a threshold value. If the return signal is stronger than the threshold, then there's something there, otherwise the space is empty. Of course the threshold has to be far enough above the normal noise level that you can be sure not to get a lot of false-positives.
But, you could also base your decision on multiple scans. A "slightly high" return value in a single scan might be noise, but if you get several "slightly high" return values from the same area then you can progressively increase the probability of there really being a target there. This way you can detect targets even if the return signal is weaker than the normal background noise.
The upside: this technique would work with any given radar technology. It's not dependent on increasing the signal, or separating the receiver from the transmitter. The downside: Bayesian statistics takes a lot of computer power.
But given the rate at which computational power increases, it's likely that this technique will be used widely pretty soon, if it isn't already.