Unlike most people on this board, I don't believe the Soviets are realy that doomed in this situation. They do have some strong points - most importantly their armed forces in 1945 are superior to those of the allies in both quality and quantity. However, I too believe that they are at a disadvantage. Starting the preparations in late 1944 may or may not help them overcome this disadvantage. In any case there are some aspects they need to be mindfull of:
1. manpower: as pointed out allready, historically the soviets were scraping the bottom of the barell in terms of reinforcements, while the western allies (especially the USA) still had a huge manpower pool. If ww3 lasts for more than a few months it's the soviets who are going to find themselves with a steadily shrinking army faced with endless wawes of enemies. A POD in 1944 may not do much good but they still need to try and keep casualities at a minimum in the last months of the war, particularly by slowing down their offensives. After all, they have allready decided to start ww3 so there is no pressure to gain as much land as possible before ww2 ends.
The secondary advantage of this change would be that the western allies will be forced to expend more men and material to defeat Germany.
2. economy: they need to reorganize the economy, especially by strengthening tose areas where they are most dependant of lend lease. Since I'm not very knowledgeable about the relative impact (not the absolute numbers) of lend lease on the soviet war-making capability, I don't know how easy this will be. In any case, it is likely to result in an even greater slowing down of soviet offensives.
3. air power: the Soviets were inferior to the westerners in exactly 2 areas: navy and strategic airforces (including high altitude fighters). If the former will probably not be a major problem, the latter will definitely be one. Allied strategic bombers will be able to do great damage even with conventional payloads, and once the atomic bomb comes online they become even more dangerous. Therefore, going to war without a sure way of stopping the bombers would be madness on the part of the soviets. While the soviet airforce of 1945 was very powerfull at low lever its ability to perform interceptions at high altitude was very limited. What the soviets need to do is to start developing a powerfull hig level interceptor. They did have such a plane in 1941 (the MiG 1) but the type was abandoned as it was of limited use on the eastern front. It should be resurected and maybe upgraded with western tech while this is still available.
Even worse for the soviets is that the jet age is coming and they have missed the train. They should start a crash program of jet fighter development and try to beg or steal as much tech from the west as possible before they go to war.
4. Allies: The soviets are again at a disadvantage because most of the minor allies are more simpathetic towards the Anglo-Americans, and most of the former axis countries feel the same. In eastern europe in particular, the soviets were hated in many areas so at least initially they will be fighting on hostile ground. They need to strengthen the puppet governments as much as possible, at least in those areas where there is some hope of success. (I rate Yugoslavia, Bulgaria and Czechoslovakia as the countries most sympathetic towards the soviets, while Romania and Hungary are the most hostile, being essentially lost causes).
Allso, a formal alliance with Japan is a must, and even in 1945 it can bring great benefits to both sides.
And finally, the USSR needs a good casus beli and needs to make it look like the other side started the war, or else morale will collapse imediately.