What affecs pings / latency / rtt ?
There are a number of factors that can cause high "ping" (latency, rtt). To name a few, heavy internet traffic at the time, congested/overloaded routers on the way to the target machine, low quality/inadequate bandwidth are the most common causes.
The first step in troubleshooting high latency is testing with traceroute. To do so, drop into command prompt (Start > Run > type: cmd), and type: tracert speedguide.net (or the host/site you're trying to reach).
The result shows every hop from you to the target machine, and its corresponding latency.
Any spike in latency along the way can usually identify the location of the current problem.
High latency is usually out of the user control, however contacting your ISP with traceroute results can greatly help identify and troubleshoot the exact issue.
Ping is not "tweakable" by the user in most cases, especially if the latency spike occurs after the first hop in the traceroute result. The only exceptions are local problems like spyware or P2P programs hogging all the bandwidth, local interference between devices, etc.
To eliminate possible interference problems, try to put your modem/router far from all electrical devices and do not coil up any excess signal or power cables.