The cost of first seeking a Locked Up Position (LUP) and then trying for a win may be too great. Anything which reduces the probability of a win should be avoided even if you subscribe to the "Surviving is better than being eliminated" theory. (There are a number of people who believe this theory is nonsense; these people are called "insane".)
A LUP should not have been attempted if to accomplish it you had to:
The second situation above tends heavily towards draws by the formation of very tight alliances which are impossible to break up. I've been a survivor in several face-to-face games in which two power blocks formed, each consisting of two countries sharing a LUP; no country was willing to attack his ally for fear of losing his LUP and being eliminated by the other power block. This was discussed at great length in one of the games and each of the major countries in the opposing power blocks agreed to attack his minor ally provided the other did so first; neither was willing to take the initial risk, consequently the game ended with four unhappy players in a four-way draw. This is the equivalent of playing "chicken": "I won't break up my LUP coalition until you do, and if you don't, you can't win." This argument is, of course, symmetric.
If another country gets a LUP, he may be satisfied with a draw and be unwilling to risk his LUP for an increased probability of a win. You can still win, but the probability of your doing so is considerable less once another country gets a LUP. If another country has gotten a LUP, you are not a "great" player. (Although you may still be a "good" player.)
Once you get a LUP, your first diplomatic act will be an attempt to get all of the other countries to attack your largest opponent (why not dream big?) in return for non-aggression from you; if pressed, offer an alliance. Point out to them that it is futile to attack you since you have a LUP; on the other hand, you are willing to assist them and they have a chance to make gains. Suggest specific tactics and show how they succeed (not everyone will be persuaded by your flowery oratory alone.)
If your largest opponent has a LUP as well, you still want to get all the other countries to ally with you -- it's just more difficult. First, no one may realize your opponent has a LUP (even himself!). Moral: if you have a strong defense which can't be broken, let the countries with whom you wish to ally know about it (remember the Russian defense system in Dr. Strangelove?). If the other countries point out to you that your opponent also has a LUP, tell them that he doesn't know it, or if he has been wise enough to have already told them (your strategy is symmetric: whatever you can do, he can also do), you can tell them that he will probable "botch" it due to poor tactical skill or a habit of missing moves. If you have to fib (contrary to popular opinion, a fib is not a lie but merely a statement whose correspondence to a real space-time sequence of events is nonexistent) a little about your opponent's skill, do so carefully and in a believable manner.
If the minor countries actually do make gains against the other major country, you can afford to sit back and be content with lining up your forces for the big push; very mild pressure should be exerted on the minor countries to give you a supply center now and then but don't press them or get greedy (it has been my experience to observe more outright wins thrown away by cupidity than by any other trait among experienced players). Continue to negotiate in detail with minor countries offering second place in return for helping you win. Be honest and keep your treaties if at all possible -- your offers will be more credible and more likely to be accepted. It requires more skill to win without stabbing so try for that kind of win. Take advantage of stabs in a game to spread distrust among the other countries. A player's stabbing indicates to me that his negotiating ability has failed him (there are some, however, who stab merely for the pleasure of it -- these people are called "criminally insane"). Honesty represents long-term policy; reputations spread fast.
If the minor countries do not make gains against your opponent, you have an excuse (as differentiated from the reason, which must always be that it increases the probability for a win -- this of course, is a simplification: some off players do it to get even) to attack them ("Everybody has to attack somebody, eh?" said with a sheepish grin, or "What else could I do?") even if it is purely offensive, e.g. Israel's "preemptive" strike in the Six Days War. You may eliminate them and outright win since their units will be deployed against the other major country.
If you haven't won yet, you have one last stratagem to employ before you must ally with the other major power: see if any of the remaining minor countries will throw supply centers to you for money. When this stratagem (or your money) has been exhausted, you are forced to ally with the other major country(s): you are in the situation that is unfortunate; everyone is against you. You may still win if you can get an ally. The worst you will get is second and that it better than a four- or five-way draw.
It was my experience once as Austria to be stabbed by Russia and France (Italy and Germany were almost eliminated) while fighting Turkey. France attacked because he knew England and Russia were very strongly allied and were going to attack Austria; he was afraid to attack England because of the alliance and thought he could get a piece of the Austrian pie. Fortunately France stabbed with a spoon and Turkey allied with Austria against Russia so that Russia ended up losing one supply center while Austria maintained nine (although they weren't the same ones since the Russian-English alliance took two). France was not one of the "great" tacticians of all time and after being stopped on the Italian front, finally agreed to attack England. Moral: