After you (very carefully) rip up the floorboards, you should clean the area near the tongue. Years of dirt between the floorboards becomes compressed above the tongue by the seasonal expansion and contraction of the floorboards. Just scrape it out with an old chisel and this will allow your floorboards to sit closer together without the old dirt in between them.
Also, some of the boards will be bowed. You can buy a special tool to force them together but you have to be careful about damaging the tongue edge of the boards you are laying down. One way to force them together is to cut a wedge of scrap floorboard with the groove side exposed. Place the groove of the wedge against the tongue of the board you are trying to force into place. Then, nail a board onto the joists on the other edge of the wedge. When you hammer the wedge it will tighten the floorboard without damaging the groove and you can nail it into plce. The toenail through the groove will also pull the board home.
Put strips of tarpaper over the joists to help prevent the floor from creaking.