I've recently lifted a trace (first time in 2 years) on a board in a PC power supply. What is the best way to repair this issue and similar issues (consumer PCBs) without something like a conductive pen or other special tools (eyelets etc.)?
You don't give enough specifics (did you just lift trace, or did you take pad with it? SMT or through-hole?). The basic idea I use for this kind of repair is to trim off the lifted part of trace and use suitably thick wire to bridge the gap. I use this basic idea when repairing old arcade equipment, so it's not complete theory. On the other hand, it doesn't necessarily come up to NASA spec.
If you lifted a trace mid-line (not at a pad), then trim the lifted part, scrape solder mask off at either end, and solder the wire directly to the exposed trace (being careful not to lift any MORE trace along the way!). After soldering, use superglue or something to tack down the wire so it can't get yanked up and do more damage.
If you lifted an SMT pad that doesn't have a via under it, then trim off the lose bits of trace and pad, and scrape the solder mask off of the end of the trace to expose it. Solder down the rest of the legs of the chip to their appropriate pads, then attach a wire to the top of the leg with the missing pad, and run it to the open end of the trace. Again, tack the wire down with superglue or something.
If you lifted a through-hole or a via on a 2 layer board, then trim the loose bits back on both sides of the board. If the hole is for a part, install the part, then solder wires to the affected leg on both sides of the board, and run them to the trimmed off ends of the broken traces. If it's a via, try to clear the via hole and get a piece of 30 gauge wire to go through (some vias are large enough for this, some are not - it just depends on the manufacturer). If successful, you'll be able to solder wire to it top & bottom just like a part. If not, then I've got no good solution other than running a wire around the board to the other side.
If you lifted a through-hole or via that leads to an interior layer of the board, then you're going to have to find somewhere else that the inner layer's signal surfaces and run wire appropriately. This requires some knowledge of the board's layout, and leaves that interior trace kind hanging out there, so it's not a really good solution, but in a pinch it can work.