DNA molecules can be concentrated in a narrow region of a nanochannel when driven electrokinetically in submillimolar salt solutions. Transport experiments and theoretical modeling reveal the interplay of electrophoresis, electro-osmosis, and the unique statistical properties of confined polymers that lead to DNA aggregation. A finite conductance through the bulk of the device also plays a crucial role by influencing the electric fields in the nanochannel. We build on this understanding by demonstrating how a nanofluidic device with integrated electrodes can preconcentrate DNA at selected locations and at physiological salt concentrations that are relevant to lab-on-a-chip applications.