How to safely raise server room humidity

I've always heard 40%, though I can't back that up. I will say though that you need some humidity to reduce static electricity build up.

EDIT:

Ah, I found my documentation, good old Sun Microsystems Part No. 805-5863-13, "Sun Microsystems Data Center Site Planning Guide: Data Centers’ Best Practices"

Temperature and relative humidity conditions should be maintained at levels that allow for the greatest operational buffer in case of environmental support equipment down-time. The goal levels for the computer room should be determined in a manner that will achieve the greatest operational buffer and the least possibility of negative influence. The specific hardware design, room configuration, environmental support equipment design and other influencing factors should be taken into consideration when determining the specific relative humidity control appropriate for a particular room. Psychrometrics can affect hardware through thermal influences, Electrostatic Discharge (ESD), and increases in environmental corrosivity.

And:

Under most circumstances, air conditioners should be set at 72º F (22º C) with a sensitivity range of +/- 2º F (+/-1º C). Humidifiers, in most cases, should be set at 48% RH with a sensitivity range of +/- 3% RH. The set-points of the air conditioners should always be chosen in an effort to maintain the optimal recommended temperature and relative humidity levels for the room environment. These set points should maintain appropriate conditions, while allowing wide enough sensitivity ranges to help avoid frequent cycling of the units. While these tight ranges would be difficult to maintain in a loosely controlled office environment, they should be easily attained in a controlled data center.

Numerous factors, such as heat-load and vapor barrier integrity, will influence the actual set-points. If the room lacks adequate vapor barrier protection, for instance, it may be necessary to adjust humidifier set points to accommodate seasonal influences. Ideally, all inappropriate influences on the data center environment will be eliminated, but in the event that they are not, minor adjustments, made by trained personnel, can help alleviate their effects on the environment.

And on Electrostatic Discharge:

The maintenance of appropriate relative humidity levels is probably the most universal and easiest means of addressing ESD concerns. Appropriate moisture levels will help ease the dissipation of charges, lessening the likelihood of catastrophic failures. The following chart illustrates the effect moisture levels can have on electrostatic charge generation. Note – Source Simco, A Basic Guide to an ESD Control Program for Electronics Manufacturers

TABLE 6-3 Electrostatic Voltage At Workstations
                            Static Voltage 
Means Of Static Generation  Relative Humidity 10-20% Relative Humidity 65-90%
Walking Across Carpet       35,000                   1,500
Walking over vinyl floor    12,000                   250
Worker at bench             6,000                    100
Vinyl envelopes for work instructions 
                            7,000                    600
Common polly bag picked up from bench
                            20,000                   1,200
Work chair padded with urethane foam 
                            18,000                   1,500

Recommendations on server room temperature vary greatly.

This guide says that:

General recommendations suggest that you should not go below 10°C (50°F) or above 28°C (82°F). Although this seems a wide range these are the extremes and it is far more common to keep the ambient temperature around 20-21°C (68-71°F). For a variety of reasons this can sometimes be a tall order.

This discussion on Slashdot has a variety of answers but most of them within the range quoted above.

Update: As others have commented below Google recommends 26.7°C (80°F) for data centres.

Also the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has recently updated their recommended tempature range to be from 18°C-27°C (64.4°F-80.6°F).

However this article agains highlights that there is still no consensus on the subject. As mentioned in the article I would highlight that:

...nudging the thermostat higher may also leave less time to recover from a cooling failure, and is only appropriate for companies with a strong understanding of the cooling conditions in their facility.

IMO most companies would not have such a strong understanding of cooling conditions and thus it would be safer in a small business environment to be running the rooms a little cooler.

NB: It is important to note there are a lot more factors to consider in a server/data room than just the temperature, air flow & humidity for example are also important concerns.