So your the Soviets. Its April 41, you see all these airfields being build in Poland and the forces marshaling up. I know by 1942 I will have the strength to defend much easier, and by 1943 should be completely safe. The logical thing to try is appeasement, would have probably worked in November 1940, but may not now.
1) Sign the November 40 treaty now, without any additional demands.
2) Return Buknovia to Rommania, Viborg and Hango to Finland and offer to give up the region around Lemberg containing oil to Germany. (None of that hurts you really strategically)
3) Offer a new economic agreement, lots of give and not much get
4) At the same time, arrange a Soviet military demonstration on the border, show off a big number of T34 tanks.
5) Show off to the Germans the size of your military and the expected size of the reserves you can generate.
6) Allow axis agent to flaunt themselves all over the Soviet union to see for themselves.
7) At the same time, tell the Germans you see their build up and wont be surprised, and will be fully prepared.