I looked over this thread, but no one seems to have mentioned my favorite way of getting LW to run as an ultrapeer, so I thought I'd outline it here for whoever might find it useful.

First, exit LimeWire if it's running and check your taskbar, task manager, etc. to make sure the program is completely shut down and not just hiding. Next, find your LimeWire config file and open it in your favorite text editor; on Windows 7 machines, this will be at "C:\Users\{you}\AppData\Roaming\LimeWire\limewire.props". In this file, locate the lines starting with "DOWNTIME_HISTORY=" and "UPTIME_HISTORY=". (One way LimeWire decides if you're worthy of being an ultrapeer - possibly the only way - is by saving stats on past program performance and examining them each time it starts up. Ostensibly, ultrapeers should be on reliable machines, with reliable Internet connections.)

If your machine isn't trying to be an ultrapeer, chances are you have a lot of small (three- or four-digit) numbers in one or both of these lines, which will happen if you don't keep your PC running for long periods of time, or if you frequently run LimeWire in short bursts, say, to look for and download one file at a time. To increase your chances of LimeWire starting up in ultrapeer mode, add digits or remove semicolons in these lines to make every number listed at least five digits long - and, for good measure, find the line starting with "AVERAGE_UPTIME=" (this is usually the last line in the file) and delete it to force LimeWire to recalculate this number.

In my experience, every time I've done this, LimeWire has gone directly into ultrapeer mode at the next run. YMMV.

As long as people with terrible connections don't misuse it, that's actually a very nice approach

Nice approach!

I tried experimenting with those numbers last year or so. It does seem to have an effect.

However I tend to have long sessions anyway. My LW will sometimes start up immediately as UP or at least after an hour or occasionally after a few hours.
Most likely because I already have good average uptime stats, I find that when I first open LW, if I continually remove the first peers that connect to me, it seems to force LW to become an ultrapeer. Again, not a recommended approach for those whom are firewalled or often have connection problems.

I find with LW 5 that I tend to get 40-50% better search results connected as a Leaf than when running as an ultrapeer. It can all depend upon the peers you are connected to and whom they are connected to of course. But I also often get less spam results when running as a leaf.
Example: 120 to 150 search replies whilst in Ultrapeer mode. 230 search replies whilst a Leaf. A concept that remains true year after year of testing this. My program is not firewalled which can play a significant role in search results obtained due to not losing UDP search result data.

Thus, the argument of connecting as an ultrapeer gets you better/more results for searches is a bit of a myth.
I recall one of the original FrostWire devs saying the same thing about 10 years ago. He made the point a leaf sees more of the search horizon than an ultrapeer.

On the other hand, connecting as an ultrapeer will help others to connect to the network.

Sharing files also plays an important part in search results. The more files you share, the more likely over time you will obtain better search results. This probably has something to do with how shared files are cached across the network and reaches out to other peers possibly beyond the standard horizon(s).